No one actually knows why the DSP modes exist. Perhaps for the same reason mold
forms on cheese. It is a kind of feature
creep that manufacturers feel obliged to
add because the others have it.

There have been a few proprietary modes that
actually add value. I had one in an older
Pioneer reciever called "DPL Cinema" which plain sounded better in my room. Yamaha has a couple like that as well.

The basic answer is, however, that DSP modes
are a triumph of technology over reason.

Eddyboy