There are two main reasons why vote. First of all, voting brings in new people - more friends for you to play with. And second of all, voting gives you awesome rewards!
Is there a reward?
Of course. Every subserver gives either direct rewards (Money, XP, TNT...) or crate keys to get a random reward each time with a chance of getting very good stuff!
How many times can I vote?
This depends from site to site. But most of the time, you can vote every 24 hours. Some sites allow 12h voting as well. Check back regularly to gain more rewards!