The one mentioned at the very end: the drain pipe the washer's drain connected to had a PVC cover that needed to be drilled/punched out (since if a washer wasn't connected it'd be a hole in that drain!). If you're replacing another washer, it was probably already punched out.
It's cheaper to make it without a hole than add a separate component and screw threads on both components. After all, you will drill the hole zero or one time, as needed.
