ORMs are essentially abstracting relational data structures to fit an OOP development style. It allows OOP engineers to access a DB without straying far from their comfort zone.
That's the sales pitch. In essence, they are rarely ever a good idea. As database drivers have become more simple to manage, and ORM's have tried to become more complex, the value proposition (which I'm loath to suggest existed in the first place) dissipated without any trace.