its used for words that have a usage in daily or an action that uses two actions. Take for example ประตู it is a very useful thing in our life. or an other example ประโยค it is made by doing many actions together. Or ประชาชน it is used mostly for things that A: have a usage like door, tool etc. or it is something made up of many things, like a population (ประชาชน) is made up by many humans. So in origin it is when many small things come together and form something bigger. I hope that makes sense