IMHO nowadays quite a lot of logic is moved into the front-end (for example "single web application"). In these cases you only need to provide some API in the back-end. But normally you need a bit more then "just" the data:

- some access control (e. g. sessions) - compute some fields (e. g. "age" derived from "birthday") - combine and filter data before transfering it the client (e. g. for graph traversals)