Fallout 2 and Fallout Tactics
You are much less likely to be knocked down in combat.
Fallout: New Vegas
This is useful against deathclaws, since they only use unarmed and they have a chance of knocking the player down. The perk is also useful against Legate Lanius because he only uses melee and also has a high knockdown chance.
Behind the scenes
There is a display error with this perk. The +5 DT will be shown in your Pip-Boy menu whenever you have a melee or unarmed (including none) weapon equipped. However, the added DT will correctly only apply against melee or unarmed attacks, regardless of what you have equipped. [verified]