Could someone please explain to me why there is the option to maually set the difficulty on some pools?
There were some early ASIC miners that wouldn't function correctly if the diff was set to a power of 2. So manually setting the diff to an odd number would ameliorate that problem.
Also rental services keep changing their difficulty requirements so you can effect that, sometimes, by manually setting a high diff.
Most pools automatically set the diff to a share rate of around 18 shares per minute. It is best to let the pool take care of it.