Why can you just brush on the spit coats of shellac instead of rubbing them on with a muneca?
You can but you run the risk of “bridging” the grain and pores in the wood. “Bridging” is a term used when materials do not flow into the grain of the wood. The shellac has poor flow-out properties which causes it to lay over the top of the grain, sealing over the pores. This will create thousands of voids which will resist filling with pumice as it is being applied. As the final finish begins to harden, the finish will tend to shrink into the pores. Wiping the shellac onto the surface will press the shellac into the wood grain minimizing shrinkage.