Building security. This was mentioned a couple of years ago in a topic where someone wanted to personally deliver a cheque to the Monzo building and was a bit annoyed that this didn’t really speed up the process (because it then has to make its way to Monzo’s postroom, and then on to their cheque facilities provider).
I’ve kept my First Direct account for paying cheques in (among other reasons). Very simple to use HSBC ATMs to scan and deposit them, which I’ve done all of, I think, twice in the last five years.