There are several reasons for tipping dealers, dealers usually earn slightly more than minimum wage from the casino they work for. And most of the money they make is in tips and betting offers and if you get a big win why can't it hurt to tip a little for a meal, tipping is not mandatory it's up to your conscience when you hit a big win.
If you are only going to tip, give it when you get up to leave and don't tip while you are still at the table or still playing.