The foam just adds some "buffer" so the blocks don't have to be "exactly" the right size. I can cut them slightly too small and the foam will compress when the monitor is pushed between them for a nice tight fit and no scratches on the monitor should you ever remove it and want to use it elsewhere.
On mine, the 2x4 braces actually take all the weight and support the monitor. The strap is just to keep it in place and make sure it doesn't flip out off the supports should the cabinet get tipped back or shaken roughly. (Pretty unlikely regardless but is cheap insurance, 21" trinitron monitors aren't exactly inexpensive!)