Basically pulseaudio creates virtual devices in /dev that mimick the actual hardware devices various apps write to if they dont allow configuration directly to pulseaudio or some other audio daemon. Thus, an app that writes to alsa directly, would have been transparently using pulseaudio if pulseaudio was setup and running.
Removing it removes this usually pointless daemon from the audio subsystem, allowing applications to talk directly with alsa, which would then no longer be circumvented by the fake device files. While pulseaudio has some uses and unique features, most people wont ever miss them.
While it no doubt helps reduce av sync issues with things like flash, the real problem with flash performance is flash itself, not the video decoding, not the audio, not the streaming, but in how flash presents these things and demuxes them and displays them in the case of video. The general idea out there is that it has something to do with drm used to protect said video that is creating the slowdown.
When you can play back 720p matroska H264 files over the network without dropping frames but you can't play back 480i flash video unless it's windowed without skipping frames then something is horribly wrong there. The problem is not in the audio system, video, processor or network and memory.