Say you are migrating password algorithms in a login system. Typical way to do it would be to add columns for new algorithm hash and ask people to change passwords once they login with the old one. Front end only needs to know to show the change password screen.
If the backend dev needs to change a field in the database, or add a new one, then can might affect the front end dev.
Also - simple things like validation rules - i.e. what can the front end dev allow users to store into the DB vs what can the back end dev use/need.