The witness scheme as you call it mine steem and help out the steem blockchain, were do you think the steem comes from? it Needs to be Mined(Witnessed) before it can be paid out to you in the from as a payment for your post.
If you were to read the whitepaper you would know exactly what a steem witness do. They are voted on by members of steemit.
The witnesses pay for the server cost to run a witness it benefits you and every one else.