Without a walking foot the feed on the machine pushes the fabric through OK, the problem is if the two layers want to slip about which makes the lower layer feed faster than the top layer. The walking foot provides another set of feed dogs on the top which push at the same rate as the lower ones (because lever connects it to the needle going up and down.
The feeds on my Husqvarna walking foot do correspond with the feeds on the machine but they are much further apart than the feeds on the 201. I used to think that that mattered, but thinking about it I can't see why it should matter. If you are pushing thick fabric through, especially a quilt sandwich with wadding in the middle, the two sets of feed dogs are not even close together. The walking foot just adds extra push to the top layer of fabric.
Think of pushing something heavy like a car. One person pulling from a towrope attached to centre front and two people pushing from the back, one each side. Each person adds to the momentum but from slightly different angles.