I wouldn't say a pressure pot is a must BUT, consider this....... When I cast my worthless wood blanks I often use wood that I want as much resin to get WAY up into the tiniest crevices and pores. In order to do this, pressure is required to push the resin up into the wood and I believe as a side benefit you get a better bond between the resin and the wood.
You can just dump some wood pieces in a mold and pour some resin in but you'll likely have voids........ that you won't see until you turn the blank down!
Mesquiteman is the expert and probably the inventor around here on this method of casting, visit his website and look at his blanks, especially the argerita root blanks and look closely at how far into the wood the resin has travelled.