<s>[INST] Why compile Python code? Why would you compile a Python script? You can run them directly from the .py file and it works fine, so is there a performance advantage or something? 
I also notice that some files in my application get compiled into .pyc while others do not, why is this?
 [/INST] It's compiled to bytecode which can be used much, much, much faster.
The reason some files aren't compiled is that the main script, which you invoke with python main.py is recompiled every time you run the script. All imported scripts will be compiled and stored on the disk.
Important addition by Ben Blank:

It's worth noting that while running a
  compiled script has a faster startup
  time (as it doesn't need to be
  compiled), it doesn't run any
  faster.

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