Some reasons to forage (applies to berries, nuts, fruits, not just mushrooms):

* Foraging is very satisfying to do

* Foraging allows you to participate in your cultural practices (although sadly many people in the US are long-cut off from such cultural knowledge)

* You can get fresher food foraging

* There absolutely are interesting varieties that aren't sold because they are hard to farm/transport/store etc.

* Foraged food is free