When you rip, you either use the fence, or draw a line on the piece and do it free hand.
Using the fence alone can give mixed results unless everything is tuned perfectly and the piece of timber is very straight to between with.
Doing it free hand takes skill.
This is allows you to work somewhere inbetween, so you're still using a fence for consistent thickness, but as Steve says, you can still apply some manual steering to counteract any small alignments issues.