What do you use the little half-moon side eye near the base of a fishing pole for?

That's there so you have a place to hook your terminal tackle during transport or storage.

Pretty sure that's correct, but it doesn't answer the question. What do YOU USE it for?

I use mine to generate stupid questions in the Pond Boss forum. Otherwise, it seems redundant.

I use it for for hooking terminal tackle during transport or storage.

Put half a dozen rods with crankbaits hooked on a line guide in the locker box on your boat and take off for the river.
You will have a snagglepuss mess when you try to get them out.