Bottle caps are used frequently as a currency because they are difficult to conterfeit and the number of caps existing are small. Bottle caps are backed by the merchants of the Hub, and can be traded anywhere. An advertising campaign that the Nuka-Cola Corporation was testing out allowed the exchange of bottle caps with robotic vendors at the Whitespring Resort due to the release of Nuka-Cola Quantum.[Non-canon 1]
Crimson Caravan Company, sought to control all bottle cap production and ensure that no one can mass produce them and inflate the currency. Controlling bottle cap presses also allows them to replace worn out and damaged bottle caps, keeping the pool of currency stable. Bottle caps, NCR dollars and Legion currency are all considered legal tender by the various caravan companies and on the New Vegas Strip. Mojave merchants also accept nonstandard variants, such as Sunset Sarsaparilla bottle caps.
Physically they appear identical to standard Nuka-Cola bottle caps and are presumably intended to be used for trading in place of actual bottle caps. They have no trade value and cannot be used for any monetary purpose.
Joshua Sawyer mentioned that the reason why the Hub chose to use caps instead of a new currency is largely the same reason why the game universe uses caps, which is nostalgia. He also mentioned that the Hub links NCR with the Mojave Wasteland and beyond, so the merchants there grew frustrated with NCR's handling of the currency crisis, conspiring to re-introduce the bottle cap as a water-backed currency that could "bridge the gap" between NCR and Legion territory.[Non-canon 3][Non-canon 4]
↑The Courier: "What makes a bottle cap genuine?" Alice McLafferty: "Lots of little things - the paint on the label, the machining, the type of metal it's made from. I know there's counterfeit caps floating around, of course. Fortunately, they're very time-consuming to make, so the numbers are small." (Alice McLafferty's dialogue)
↑The Vault Dweller: "Tell me more about bartering." Katrina: "And if you don't have enough items to trade, there are always caps. Bottle caps are the only common money found out here. The caps are backed by the merchants of the Hub, so you can trade them anywhere." (Katrina's dialogue)
↑Whitespring concierge: "To celebrate the release of Nuka-Cola Quantum, The Whitespring has partnered with the Nuka-Cola Corporation to accept bottlecaps at all of our shops. For a limited time, you can take advantage of some truly amazing deals if you have the caps." (Whitespring concierge's dialogue)
↑The Courier: "Are new bottle caps ever made?" Alice McLafferty: "Certainly. Bottle caps do wear out or get damaged. Some people even insist on using bottle caps in explosive devices for some reason. We make it a point to scour Pre-War bottling plants and recover or disable the bottle cap presses. It seems we missed one." (Alice McLafferty's dialogue)
↑Fallout: The Roleplaying Game Rulebook p. 84: "The standard accepted currency across North America, bottle caps are finite in supply and difficult to reproduce, which makes them ideal as a token of value for merchants travelling the wastelands, in the absence of the banking methods that supported pre-War currencies."
↑Scott Campbell: "I remember my fellow Fallout designer, Brian Freyermuth, asking how much something will cost in a shop. I remember thinking, “cost what?” What was our game currency? We went through a few ideas: A pure bartering system? Nah, that would be difficult for the player to understand the worth of anything. (Two molerat pelts for a cup of coffee? Is that good?) Bullets as the currency? I gotta admit, bullets are definitely useful in the wasteland. But that idea was shot down (sorry) when we realized that people would be very hesitant to use things like machineguns, since every trigger-pull would directly lower their bank accounts! That level of financial restraint wouldn’t be enjoyable. Credit cards ? – just the hard plastic cards, of course - but most would have probably been melted in the nuclear firestorms. So, I thought, what shiny token-sized thing would you find strewn around the trash piles? Something common, but not so common as to be everywhere? Bottlecaps, of course! (That, and I liked the idea of a string of caps on a chord that jingled when people pulled them out.)" (Origins of Fallout)
↑Joshua Sawyer on Something Awful Forums: "It happened during the BoS-NCR war. I believe Alice McLafferty mentions it, but I'm not positive. She doesn't detail the events in this much detail, but here they are: The attacks caused NCR citizens (and others who held NCR currency) to panic, resulting in a rush to reclaim the listed face value of currency from NCR's gold reserves. Inability to do this at several locations (especially near the periphery of NCR territory where reserves were normally low) caused a loss of faith in NCR's ability to back their currency. Though NCR eventually stopped the BoS attacks, they decided to protect against future problems by switching to fiat currency. While this meant that BoS could no longer attack a) reserves or b) the source of production (all NCR bills are made in the Boneyard), some people felt more uneasy about their money not having any "real" (backed) value. This loss of confidence increased with NCR inflation, an ever-looming spectre of fiat currency. Because the Hub links NCR with the Mojave Wasteland and beyond, the merchants there grew frustrated with NCR's handling of the currency crisis. They conspired to re-introduce the bottle cap as a water-backed currency that could "bridge the gap" between NCR and Legion territory. In the time leading up to the re-introduction, they did the footwork to position themselves properly. If some old-timer had a chest full of caps, they didn't care (in fact, they thought that was great, since the old-timers would enthusiastically embrace the return of the cap), but they did seek to control or destroy production facilities and truly large volumes of caps (e.g. Typhon's treasure) whenever possible."
↑Joshua Sawyer: "How do they account for the fact that they have no control over the number of caps in circulation and any one can come in from outside the area with a huge stock of caps and screw everything up?
Hub merchants like the Crimson Caravan hire people like the Courier to deal with it. Also, there's not a big difference between someone finding a bunch of caps and finding a big chunk of gold. Loss and inflation are only a concern if the difference is large and there's a rush on the reserves. Even when US currency was gold-backed, the government continued to inflate currency numbers. This inherently devalued every previously-extant dollar relative to its backed resource, but they did it anyway because the volume change was usually small.
The reason why the Hub chose to use caps instead of a new currency is largely the same reason why the game universe uses caps: nostalgia. Remember caps? Before NCR hosed everything up? You may even have some caps still sitting around from dad's old stash. Well, that's money again. Hell, if you want to come down and turn 'em in, we'll give you water for them right now. Yessiree, the good ol' days are back again." Something Awful Forums)