Why should I use `mod_wsgi` instead of launching by python?
Since I answered your other question regarding Flask, I assume you are referring to using Flask's development server.  The problem with that is it is single threaded.
Using mod_wsgi, you would be running behind apache, which will do process forking and allow for multiple simultaneous requests to be handled.
There are other options as well.  Depending on your particular use case I would consider using eventlet's wsgi server.
I was wondering what is the advantages of mod_wsgi. For most python web framework, I can launch (daemon) the application by python directly and serve it in a port. Then when shall I use mod_wsgi?