A mech army allows you to deal with mission objects quicker than on foot (sometimes you may never reach them, getting to the other side of the board for example), in addition, as cher mentioned, make use of flamers in a more effective manner (12" move, disembark 2" and fire up to 8", better than move 6") It also gives good protection against those faster armies (DE) as you won't be so liable to being assaulted and you can almost match up to their speed so they don't dominate the feild.