Well, ‘need’ is such a difficult word to deal with. The short answer is that they ‘need’ to do so because the law of the states within which they operate requires them to do so. That, of course, explains nothing of the reason for those requirements.
This isn’t quite accurate. As a user of a non-commercial vehicle, I might well use the roads of more than one state for profit (e.g. travelling salesmen). Nevertheless, I won’t be required to have a license plate in each state - indeed I will most likely be PROHIBITED from having one in each state.
The reasoning is more complex. Partly, it has to do with assumptions about commercial vehicles vs. non-commercial vehicles. It also has to do with agreements among the states and with the federal government about how to deal with ‘interstate commerce.’ Mostly, it has to do with the fact that each state wants to make as much money as it can off the fees generated by taxing commercial vehicles which use their roadways.
I doubt any answer could give you the ins and outs of a decision by a state as to when exactly to require a licensing by a commercial vehicle. Policy rarely is simple.