Why do banana plugs need these holes?

Does this banana plug picture help you understand why: -

You can get different variants too: -

And end-stacking types: -

Your banana plugs have holes so that the plugs can act as banana jacks.

You plug one banana plug into the plug panel of, say, a power supply. You can then plug another banana plug into the first plug.

It's a helpful feature for making multiple connections with banana plugs.