I don't know what's so special there and why you have to be 21. ???
Vegas is Sin City. You get drunk when you wake up, spend all your money on gambling, go to strip clubs, order legal prostitutes, and party all night long.
Pretty much all of those activities require you to be 21, some strip clubs and prostitutes can be done 18+. I actually slept all day in Vegas, so of those activities I only actually got drunk when I woke up and stayed drunk the whole day, but those are just a few of the things that are common to do in Vegas.