I use it to prevent chafing from clothing, and rubbing from contact with seat while pedaling. I like it because it goes on easy and soothing to the touch, it does not cake or itch. Also easy to remove with soap and water and is safe to use with my sensitive skin.
This is a standard piece of equipment for triathlers, probably because it works well and lasts a long time. It's one of those things that you don't really notice when it's working but you'd definitely notice if you forgot to use it. Besides the normal anti-chafe uses, I use this as a zipper lubricant (especially useful for 45NRTH's boots).