Try to establish a communication between two Uno's with a single Uno! Then you will quickly notice the advantage of having more than one Uno.
I also own several Uno's, 1 Mega, 4 - 5 Nanos, 2 Pro Minis (5V + 3.3V) and a ProMicro (32U4). As well as some Attinys. So I can, for example, work on several projects or project parts in parallel and, if necessary, also build up the hardware of a forum user who has a problem and help him because I can test the code in a practical way.