ALSA stands for Advanced Linux Sound Architecture, I'd encourage you to poke around their project website if you're truly curious. Specifically I'd take a look at the "I'm new to ALSA pages & tutorials.
The ArchLinux wiki probably describes it the best.
The Advanced Linux Sound Architecture (ALSA) is a Linux kernel component which replaced the original Open Sound System (OSSv3) for providing device drivers for sound cards. Besides the sound device drivers, ALSA also bundles a user space library for application developers who want to use driver features with a higher level API than direct interaction with the kernel drivers.
This diagram is also helpful in understanding where the various components, ALSA, JACK, etc. fit with respect to each other & the kernel.
And finally one more excerpt - How it works: Linux audio explained:
When it comes to modern Linux audio, the beginning is the Advanced Linux Sound Architecture, or ALSA. This connects to the Linux kernel and provides audio functionality to the rest of the system. But it's also far more ambitious than a normal kernel driver; it can mix, provide compatibility with other layers, create an API for programmers and work at such a low and stable latency that it can compete with the ASIO and CoreAudio equivalents on the Windows and OS X platforms.
So the bottom line is that ALSA is the layer that provides other audio software components access to the kernel, so to answer your question, yes you need it.