I always thought it was a pointless thing to do, but I still understand it. "You're going to meet with God, shouldn't you look your best for Him?" The answer to that is that God doesn't care about what clothes you're wearing. Indeed, that would be a pretty shallow standard for God to be working under...
The scriptures say, "The Lord does not look on the outward appearance, but on the heart."
The whole point of people getting really dressed up for mass is so they look as respectable as possible. Of course God isn't looking for who is in the best suit or who is wearing the biggest pearl neglace. Many just aim higher so that they don't become too laid back and allow their child to come to mass in a miny skirt.
Tonight, for example, I'll wear jeans, a polo and maybe a hoddie. It's decent looking and that's what matters. My church doesn't expect me to pull out a shirt and tie every weekend but they also don't want me walking in with a white beater on and a big chain around my neck.