Simon Fell > Its just code > ASP.NET Impersonation

I'm trying to make sense of this statement in the docs

Only application code is impersonated; compilation and configuration are read as the process token. The result of the compilation is put in the "Temporary ASP.NET files" directory. The account that is being impersonated needs to have read/write access to this directory.

If the compilation is done with the process token, then why does the account doing the impersonation need write access to the directory ? what am I missing here ?