The band makes it easy to put the harness on, but also stops the hook rubbing/wearing your wetsuit - well most of the time anyway.
Agree, precisely put - and I think this is the only reason
To hold the harness in place so easy to put on, and most of the time protecting too !
I hate those (often bulky) "bar pads" - and I always remove these, for me they are just in the way and "extra".
So without a bar pad, the spreader bar will be a nuisance if you had no cummerbund.
With a cummerbund, it is only very rare the spreader bar rides above and might scratch your stomach (or wetsuit), so no biggie and I prefer it simple like that.