My father-in-law does cabinetry and explained it to me like this:
When you paint, the paint has to have something to hold on to. So, for instance, if you just slapped latex paint over old lacquer, it likely wouldn't hold very well, if at all. Primer helps by bonding to the old surface and being bondable to the paint.
Sanding cuts holes into the surface. It increases the primed area for the paint to hold on to and improves the bonding. I can speak from experience when I say it really does help.
100 grit is a bit much tho. I would suggest something more like 220 (most sanding blocks come around this grit). 100 would be if I were priming something else. For instance, I had an old bed frame that was just stained and lacquered. I sanded it with 150, primed, sanded 220 and then painted a final coat. It was very nice after I finished.