Why would one use accelerators with fastcgi for PHP?

[fallin]

I'm a newbie to web technology, and still on a learning curve.Heard that, fastcgi would keep the compiled(interpreted) php code in memory, so why would one has to use op-code caching (apc or eaccelerators) for PHP? But I never heard of any such accelerators for Python. I'd expect as python and php are both interpreted language, it makes me think that, there has to be a room for python accelerators ? pls correct me if I'm wrong. Many thanks