What is the reason for using gambits on defence? As I understand it, gambits are used in aggressive play, such as in the King's Gambit and Queen's Gambit ...

But:


  What is the reason behind playing gambits on defence, such as
  in the French Defence?
The fact that an opening has the word "Defense" in the name usually signifies nothing more than the fact that it is a move by Black that leads into that opening. For instance, if White plays 1.c4 it is simply called the English Opening. But if, after 1.e4, Black plays 1...c5, then we reach Sicilian Defense positions; or if Black plays 1...e6 instead it is a French Defense. But the only reason the latter two are called "defenses" is because it is Black who chooses to enter each one, and the first-move advantage for White means that in the early stages of the game, Black is "defending" to some degree. This is the same reason why, for instance, when Black plays a certain setup it is called the King's Indian Defense, but when White does the same it is called the King's Indian Attack. The word "defense" in the name of an opening generally means no more than that Black has the choice to enter into it.

But Black's motives when playing one "defense" instead of another vary as widely as chess players do, and many times Black is striving to take the initiative away from White immediately. The Sicilian Defense and the King's Indian Defense, for example, both have rather aggressive reputations. Given the above explanation regarding the word "defense" in the names of openings, your question as stated boils down to, "what reasons does Black have for gambit play?" And to that, I would just say that gambit play has the same pluses and minuses for either color.