HID for bikes?

I dont really understand the point of gettin an HID kit. I know they are alot brighter and all, but why not just change your headlight bulbs. i changed my stock bulbs to the sylvania silverstars, and i can tell a pretty big difference in them for only like $50. I think they burn at like 4000 lumens, and are plenty bright enough for me.

I just put them in my r1, and havent really had a chance to ride it much cause of winter. I had them in my gixxer, and never had a problem. They lasted for the whole time i had the bike (a little over a year).