MPE is very useful if you want real time per-note expression and pitch control. (Basically, the less like a piano your controller is, the more useful MPE becomes.)

It's possible to do that in plain MIDI 1.0 by using a separate channel for each note and using CCs, channel pressure, and/or pitch bend (that's basically what MPE does), but trying to get that to work with a wide range of synths is quite difficult because every synth implements MIDI a little differently. MPE standardizes that use case so it's much easier to have something that "just works".