What does resetting the PMU do?
This Apple support article has a link that explains what happens when the PMU is reset. Three things are reset:

Speaker volume
Screen resolution
Startup disk selection


After resetting NVRAM or PRAM you may need to reconfigure your settings for speaker volume, screen resolution, startup disk selection, time zone information.

and this list (may not apply to all Macs):

Status of AppleTalk
Serial Port Configuration and Port definition
Alarm clock setting
Application font
Serial printer location
Autokey rate
Autokey delay
Speaker volume
Attention (beep) sound
Double-click time
Caret blink time (insertion point rate)
Mouse scaling (mouse speed)
Startup disk
Menu blink count
Monitor depth
32-bit addressing
Virtual memory
RAM disk
Disk cache


Another support article provides additional information:

Some information stored in PRAM includes:

Display and video settings such as refresh rate, screen resolution, number of colors

Startup volume choice

Speaker volume

Recent kernel panic information, if any

DVD region setting
Note: Mac OS X stores your preselected DVD region choice in PRAM for easy access. Resetting PRAM does not allow you to change the DVD
region.