In the beginning of time, when there no internet and not so many computers. The synth manufacturers saw it nessecery to make easier to program synthesizers.
Many types of synthesizers had these programmers as the JX-3P, JX-8P and even the D-50 had a programmer. Other mnufacturers had it also the Yamaha DX7 had a programmer that looked something like a airplane cockpit.
I dont know what the PG-200 can do, but probably make programming easy.
You say that it is used with a SH-101, then I am not to sure that this is a synth programmer, it may be a form of sequencer. But if my memory dont let me down, all the sequencers had another name like, MC pr SQ.
But I dont know, maybe someone else knows.