You are correct that end of the day batches then happen to settle all those auths into actual sales - but that's so you can do things like instantly refund a customer for a cashier fuckup/etc. and not get charged the discount fee both ways like how a regular refund works.

There are no stores simply accepting any/all magnetic swipe transactions and then only at the end of the day figuring out that oops, that card didn't have enough credit available after all.