Since version 8.04, Ubuntu has used PulseAudio as its default sound system. After hearing of various problems people have had with PulseAudio (like this one and this one), one may wonder why Ubuntu uses PulseAudio at all, especially since these problems can often be fixed by turning off PulseAudio with no ill effects. The rationale for the switch to PulseAudio in Ubuntu is laid out on this page:
I’m providing this link in the hopes that others don’t have to search as far for the answer as I did. Here are some highlights from that document, describing the benefits of PulseAudio:
Beyond the obvious sound mixing functionality it offers advanced audio features like “desktop bling”, hot-plug support, transparent network audio, hot moving of playback streams between audio devices, separate volume adjustments for all playback or record streams, very low latency, very precise latency estimation (even over the network), a modern zero-copy memory management, a wide range of extension modules, availability for many operating systems, and compatibility with 90% of all currently available audio applications for Linux in one way or another.
The document also has an extensive list of Use cases, which demonstrate where PulseAudio can be useful. While PulseAudio has many interesting features, the majority of these are not exposed to the user through a discoverable user interface so people don’t know they exist and, thus, don’t miss them if they disable PulseAudio.
For those that are interested, I found the above wiki page by searching for Hardy Heron release notes, leading me to this page, which linked to this blueprint, whose full specification is the wiki page.