Using multiple outs from a single plugin is useful when you want to apply different processing to different sounds (typical example is a drum kit, kick and snare usually benefit from treating them individually).
Similarly you might want to use separate MIDI tracks for different sounds, just to make editing easier.
For something like a sampled piano, you only really need that one out/track. Of course you can do all sorts of fancy parallel processing and doubling tricks with track routing, but that's special needs territory and getting good results requires quite a bit of experience.
In the end we're free to use the program in any way that feels good for us, whatever is fast, effective and gives the results you want is the right way. There's usually several ways to do most things in REAPER, pick one that works for you and focus on the music