Having been enslaved to many Seattle games while Marshawn Lynch was playing, I watched him start slow many times.
But on each run the OL players had an easier assignment than when pass blocking, and the defense overall had to work harder, and get hit harder. The receivers get to block - they have work to do, but they're not doing the exhaustive work of making three abrupt direction change moves at high speed while sprinting 40 yards.
Over the course of a game the effects are cumulative. The run game becomes more effective. When you need three yards at the goal line late in the game, there's a difference between fielding a team that has only run ten times and one that has run 25 times if the run is working at all.
Williams hasn't really been tried yet.
He's been in the games for a lot of snaps, but fewer touches than his snap count would suggest. As he hasn't carried that much I wouldn't worry about his snap count. He was, according to reports, a workhorse back in college. He's a force in pass protection. He needs to settle down, get in better sync with Stafford as a receiver out of the backfield, but he's quick and shifty once he gets the ball.
That's before we talk about the notion of splitting his carries, keeping fresh legs in the game, or getting a proven power back in on short yardage situations.