If folks use banks they use different parts. When you switch banks you switch parts. If folks that don’t use parts aren’t filling up 16 patterns in a bank might as well assign the other patterns to a different part or more. It’s the same as switching banks where you can assign different samples and fx and whatnot just without switching banks. You can fit more stuff per project…
Instead of complete changes of samples and whatnot they’re also good to copy your first part to the second and make less drastic changes, copy second to the third and make more changes, etc… Lots of uses really…
They’re a fundamental “part” of the OT and like others have said your always using one and again when you switch banks your using another, you can use up to 64 per project instead of the 16 you’d get out of using one per bank…