It helps to keep the engine clean.. and I've read that it helps direct air flow through the radiator. That said, I haven't had one on my car in several years. Eventually I'll get around to fabbing up my skid plate to go there instead.
On bugeyes, you don't have to remove it to change the oil, so I don't touch it. The only time I've taken it off was when I scoped out the uppipe install. I would like a real skid plate there, but the stock one has never bothered me.