I tried it once. Decided it was NOT for me. Cut myself once, and said never again. My wife said she didn't like it anyway. She said it made me look like I was a really well hung 12 year old, and it was too weird. LOL
From what I understand, a lot of guys do it because they like oral from their women. This helps to keep the pubic hairs out of the womans mouth.
There is also another train of thought on the matter. Some guys do it because it makes them appear larger than they actually are.