I see two reasons:

One is that a person has a nice gun of whatever gauge that they really want to use, and realizes they don't need the full gauge-correct load. They opt to skimp on recoil and save a bit of money if they reload.

The second is the person who sees it as a challenge to accomplish the most with the least, e.g. using a .410 in the field, 7/8 oz for trap, etc.