Why You Would Use App Domain?

There are 3 reasons:

1) To dynamically load and unload assembly. You can load an assembly, but you can't unload it because the CLR cannot keep track of all dependency of assembly. One workaround is to load these assemblies into a different app domain and unload the app domain once you are done.

2) For security reason. You can apply CAS to app domain to secure it from doing dangerous thing.

3) Different configuration setting. You can have different setting for each app domain that make it behave differently.