In order to be successful in pro-wrestling, one must have a character and a purpose. I mean, how could any promoter expect anyone to get the audience behind them unless they know what that person is fighting for?
Of course, having a gimmick doesn't make someone inherently more successful than those without one. For example, Bret Hart's gimmick (if you can call it that) was "Canadian guy who was good at pro-wrestling," and he had a much more fruitful career than Skinner. Or The Berzerker. Or The Red Rooster. Or Mantaur.
Okay, maybe gimmicks do more harm than good in many cases, but they do give talent an opportunity to get themselves more over than they previously were without one. Case and point - The Ringmaster became Stone Cold Steve Austin. Enough said.
Anyhow, with only a handful of original ideas left, sometimes wrestlers have to recycle old gimmicks or integrate pieces of someone else's character into their own. 99 times out of 100, these attempts are completely worthless. Very occasionally, someone manages the impossible.
Interesting to note, however, is that it often doesn't matter who used a gimmick first - it always matters who used the gimmick most effectively.