Clearing the board is a very important in the game of Crashed! Not only do you get a ton of points, but you move up a level. Also, you get special bombs after you have cleared the board one or three times (and are on a high-enough level). See Basic Game Play for more information on special bombs.
Besides using special bombs, the best way to clear the board is to let your bombs accumulate, and then click color bombs as fast as you can. This should clear the board if you're fast enough.