When do we use/download PS, PSP, and Image Pack brushes in photoshop?

Answer 1

When you need them of course!
Sometimes you need a hair brush, or a brush that is not in the default set of brushes in PS, you have to download them for example from:
http://community.livejournal.com/100x100_brushes/

and put them into this path:

C:\Program files\Adobe\Adobe Photoshop CS2\presets\Brushes\drag your brushes here

If it's a zip file unzip first