Don't underestimate the argument of "just to be on the safe side". Someone running a business who doesn't know a lot about cookies will often just put a banner on as an easy arse covering mechanism even when not required.
If the banner's not required, it ends up saying stuff like "we'd like permission to share your data with 0 partners". Ever see that?
The fact is most website operators want to use your data for non-essential purposes.