With Retention, the duration of the skill magazines lasts for 3 minutes instead of 1 minute. This perk is useful when supplementing a weapon skill with magazines, where time may be a factor. Non-combat skills, like hacking terminals or dialogue successes, generally don't need the added time.
Magazine durations, however, are for in-game time equivalents to real time, so a prolonged duration means that one can actually fast travel to different locations and still retain the effects of the magazine, such as Salesman Weekly. This effect is more pronounced with the help of something like Logan's Loophole.