Usually these are applied to motor drivern loads like macerators, windlasses, SW pumps, and the like, that can overload in normal operation, unlike other loads like lighting & electronics, so as not to need a big stash of fuses around, and teh delay in spending time to find it and replace it.
Getting the right size is important, if they tell you the next size up will work fine, tell them thanks but no thanks.
I'll check the Freeport Marine catalog when I get home tonight.