When Signal asks for a PIN, it says it's NOT needed when you start the app. So, what's it for?

Good question! I don’t know.

I almost rage-quit using Signal when they started that nonsense. I finally relented and entered a PIN (which I’ve forgotten). But it stopped requiring it…

Now I just have about 15% of my screen in Signal showing the “Verify your Signal PIN” UI, which I’ve trained myself to ignore so hard that I almost never even notice it’s there.

See my answer to your earlier question.

The PIN is just one way these platforms leak supposedly secure info. But even without that weakness, Signal and all the rest are a joke in terms of real privacy.