In your ALSA example the lower level API doesn't prevent you from mixing audio, it simply isn't as easy because you must implement it yourself, this could be ar

In your ALSA example the lower level API doesn't prevent you from mixing audio, it simply isn't as easy because you must implement it yourself, this could be arguably more flexible as well. Abstractions can hide features of the hardware but they cannot create new hardware. Whatever the abstraction is doing the client software could do instead with the lower level API. So I would say abstraction layers are often added to give ease of use, sometimes at the cost of performance and flexibility.