Some DBs have inherent limitations and you need to design something to get around them. Sometimes you have many apps and you don't want to reinvent the wheel to support what are otherwise standard accesses of a database. Sometimes it's a "special" database which makes this more convoluted, or perhaps you need to implement things like a gradual back-off retry to prevent swamping the backend. Sometimes you don't want to have to patch every app that uses the database if a database change is breaking. Sometimes you want to prevent developers from abusing your database, provide more fine-grained access control, prevent poorly researched queries which can kill performance, etc. Sometimes you're talking to more than one db.

Personally, I would start writing the abstraction as soon as I plan to have more than two apps.