Why do we need witnesses on a decentralized platform ?

Do they introduce themselves properly?
Do you know what their...
powers ?
responsibilities ?
rights ?
duties are?
They all seem to be around a little longer...
But except saying, that we should
vote for them as witness, they don't
say much about their "role".
Well ups , i read an older post, that was a
little worrying...
They will hold a big power over whole
SteemBlocks...controlling the
whole currency !?
Is that the non-violence philosophy
of Dan Larimer?
Well, i don't know !?
But feel free to teach me...