It's the basis currency for blockchain, like including rewards + fees (although through currency pools you can pay fees with some tokens too)

Bitshares lets you vote and help fund and change the bitshares network through workers (there's stuff on that in original post) or even just select a proxy to vote for you if you are busy.

Bitshares was the first DAC or more commonly now called DAO, before Dash and Ethereum. The entire blockchain works through delegated votes for witnesses as well. http://docs.bitshares.org/bitshares/whatis.html

You can also use BTS as collateral to borrow smart coins like bitCNY or bitUSD into existence which are kept at CNY or USD value through smart contracts, redeemable for that much worth of BTS by settling by anyone in their possession.

There's fixed number of them total as well.

Hope that helps.