The only reason is to see definition. The bodybuilders on stage use bronzers which are made for competition and get them extremely dark. You don't need to go to that level, just hit the local tanning beds and use some lotion for a nice tan.

I'm a pretty light guy naturally so I find I really need to tan to look good especially during winter and into the spring. I was against tanning for the longest time, but you would be surprised at the amount of guys at these places. It is also a "hotbed" for beautiful women :P