It is different because 1 project represents a whole device. For example recently I planned to design a mini transceiver which will have 3 boards. Those boards should connect to each other like a sandwich.
Currently in order to maintain connections between boards I have to close and open different schematics and switch between projects all the time. For example if see that some part doesn’t fit on one PCB and I can move it to a different one, I have to make a lot of steps to move it.
There is more reasons. For example common BOM. If we have 3 projects, we basically need to generate 3 BOM files, combine them somewhere in Excel and get totals after that.
I understand, this is complicated task, because it needs some initial research because boards could be connected to each other though pin headers, ribbon cables, regular cables, gold fingers, some hex nuts or just soldered to each other. I am sure there is more options and it is difficult to define a “borderline” connections.
The main advantage of one project is integrity - we have everything in one place, we don’t need to jump between projects etc, rebuild annotations etc, check rules, make sure that we didn’t miss a pin etc.
It is worthwhile to see the device as a one project from logical perspective.
Also it doesn’t feel right when we build one thing to make several projects for it.