Say your using something like xmms or other application that has it's own volume control.
So you have the Gnome volume control stuff. It's the icon in your task bar and the mixer control stuff in your applications menu. This provides a 'sanatized' way to interact with volume controls. So you can have that nice little icon and the sound mixer buttons on your multimedia keyboard and whatnot.
Then you have the low-level Alsamixer interface. This reflects the actual hardware capabilities of your sound card. When your mucking around with the PCM slider your interacting with the sound card's hardware. It's confusing to use and each sound card has different capabilities so it's a UI that will change depending on what sound card you have.
Then you have your 3rd mixer interface at the application level.
Then on top of that people using desktops will have another volume control on their speakers. So that makes 4 different volume controls that interact with the sound card. Different applications will go through the Gnome stuff, interact with alsamixer directly, or have their own controls. Depending on how the application is configured it can interact with controls differently at different times. (ie, some apps can be configured to use OSS vs Alsa vs ESD vs artsd vs etc)
This is confusing.
Now say your trying to do a voice recording or interact with a VoIP application. Things get _very_ confusing.
With PA you have the master controls and the application controls in the same spot. Then that eliminates the requirements for mixing controls in your browser, file manager, your terminal's beeping, etc etc.
So this _should_ lower the mixing controls down to 2 on the software side. Alsa stuff should setup correctly, but it won't be for the significant number of people. So you still need alsamixer for getting the "baseline" set. But after that you should only need to deal with one interface.