In the US, most consumer banks offer online bill paying. It's convenient for me, but what's in it for the bank? Many vendors take payment electronically, but some require a paper check sent via the post office. The bank does not charge either the sender or the receiver for this service. On a small $20 bill, they must be paying a few percent in postage!
Is it cheaper than processing the personal check that I would otherwise write? I might believe that they offer this service to compete with other banks for my business, but they actively encourage me to use it too. So there must be something in it for the bank as well.
(This is thinking from the bank's perspective rather than mine, so apologies if it's off topic.)