I can think of 3 reasons. Some do it because they think they have a small dick and need to enlarge it to get a "normal" sized dick. Others do it because they simply wan't a "healthier penis", meaning improving the libido, better erections etc. And then there are those who have average dicks but want to be more than average (and possible huge). Me, I'm in it for the latter reason, I think it's just male competitiveness.

Granted, some people might have more than one reason.