You surely are correct, but it doesn't help me as a user to know where the item will come in the following situation:
That's why I suggested it. Maybe I could've used a better example as explained in my OP.

That's not exactly what I meant: I would love to have the white square show where my items would land as a QOL.

For example, when a inserter drops into a belt that moves away, it drops right. If the belt moves under the inserter (and it drops into the tunnel piece), it drops left. Just knowing that ...

TL;DR

Mark drop location for inserter on belt

What?

Why?

It prevents confusion, primarily when belt goes away/towards an inserter.
To prove my point, I noticed that my paint art has the location wrong, should've been right instead.