[n.s.]

They do it because lots of weak players like to color up some of their stack to "lock up" a win, but then they invariably have to break them back down when they start to lose again. When some idiot starts calling $18 bets with a $100 chip, it slows the game down immensely as the dealer has to count out a ton of change and may even run out of chips in his rack. I get annoyed enough at the players that do this with $25 chips ($100's don't play in my game either).

As others have said, don't color up in a limit game.