It seems he just revealed them.
Chris Haynes of Cleveland.com:
Williams will provide the Cavs with a veteran option behind Kyrie Irving at point guard. This way, they won’t be stuck with relying on rookie Kay Felder if the No. 54 pick isn’t ready to join a contender’s rotation.
Williams is due $2,194,500 this season. I don’t think this is the case, but if he is trying to extract a buyout from the Cavaliers by threatening to grind through the year rather than preemptively retiring (which would take his salary off the books), this could be a step. They need to believe he’d play to pay him a partial salary in a buyout. Again, I take Williams at his word about playing another season. It’s just hard to square his reported lack of communication with the team prior.
And though I believe Williams intends to retire after the season, next June is a long way off. He wouldn’t have to look far to find someone who changed his mind about retirement.