The gimme as defined by the The Golf Expert seems straight forward enough.

gimme - (aka: "gimmie") a putt of sufficiently short distance that the players in a group agree that it would certainly be holed and as such do not require the player to putt out (but do count one more on the score card). 

And to be sure, it certainly helps to move a group along a bit more swiftly. Don't you hate waiting for a typical Saturday foursome that meanders around the green in front of you lining up two foot putts as if they were touring pros? This is where the gimme earns it's well deserved practice.

And then there is the art of giving and receiving gimmes. There is often a certain cadence associated with the gimme putt allowance. If a lengthy putt is reasonably well executed, a gimme can come almost automatically. If, on the other hand, the execution is questionable, there may some hesitation with the gimme call which often triggers the gimme anticipation dance. The putting player feels the need to give the conceding group a bit more time to deliver the gimme call. In these cases the golfer generally takes his time approaching the ball and may casually glance up for the briefest of eye contact with his opponent. He may also slowly gesture toward the ball as if to mark with the anticipation of the "pick it up" comment.

In the end, it's a reaffirmation of personal comradery that compels opponents to give away strokes to their fellow players. Check this cool story recounting the famous two foot concession Jack Nicklaus gave to Tony Jacklin in the 69 Ryder Cup. Jack is the man!