A WSGI middleware component is a Python callable that is itself a WSGI application, but may handle requests by delegating to other WSGI applications. These applications can themselves be WSGI middleware components.

A middleware component can perform such functions as:

• Routing a request to different application objects based on the target URL, after changing the environment variables accordingly.
• Allowing multiple applications or frameworks to run side-by-side in the same process.
• Load balancing and remote processing, by forwarding requests and responses over a network.

A traditional web server does not understand or have any way to run Python applications. 

Therefore the Python community came up with WSGI as a standard interface that modules and containers could implement. WSGI is now the accepted approach for running Python web applications.

As shown in the above diagram, a WSGI server simply invokes a callable object on the WSGI application as defined by the PEP 3333 standard.

Some examples of WSGI servers are: Green Unicorn, uWSGI, mod_wsgi and CherryPy.

Why use WSGI and not just point a web server directly at an application?

• WSGI gives you flexibility. Application developers can swap out web stack components for others. For example, a developer can switch from Green Unicorn to uWSGI without modifying the application or framework that implements WSGI.
• WSGI servers promote scaling. Serving thousands of requests for dynamic content at once is the domain of WSGI servers. WSGI servers handle processing requests from the web server and deciding how to communicate those requests to an application framework's process.