Interview with Lennart Poettering (LinuxFR.org)

It matters because it means that you need at least two different audio APIs: "bare" OSS hardwired to talking directly to the kernel representation of the audio hardware, and something with a higher-level abstraction that allows to pass through a sophisticated user-space processing framework.

ALSA gives you an interface that behind your back can do both: talk to bare-metal hardware (if for some reason you want to), or have it transparently rerouted through PA. This means that ALSA provides a useful abstraction, while OSS does not at all. (Compare to e.g. OpenGL direct/indirect rendering, its hardware abstraction and what you gain over using libdrm directly).