As far as I know the POW mining on STEEM was phased out a few hard forks ago.
Basically a witness is a community elected individual that runs a server with the STEEM daemon steemd running on which creates and stores blocks for our network. Anyone can run a witness server however the community must vote you into a block producing role via the witness votes. :)
Why you became a Witness?
I became a witness to better use / sharpen my skillset, serve the STEEM community and offset my development costs of STEEM services I've got under development. :)
Thank you for your question. 🍹🍹🍹🍹