Some abstractions are good, but I've seen many projects where an abstraction layer or interface is added that does the exact same thing as the code behind it. Or even more often, there will be one or two specific functions in the layer that does actual work, but it would've been fine to just write a helper function for that and not wrap everything else. It's actually pretty rare that whatever needs to be abstracted covers an entire conceptual area to the point where thinking of it as a "layer" makes sense.