And sometimes it's not welcome :-(
I was working on a BIF (bird in flight) set up yesterday where I wanted a focus point set in mid-air where the bird would be. I put a piece of cardboard on a stick in that spot, focused, switched to MF, shut off the camera, set everything up and turned the camera back on. Many, many OOF photos later I learned that when you turn the camera on, the lens focuses to infinity even when in MF. Not a very productive day. Will try again when it quits raining.
TF