Why should one use a http server in front of a framework web server?

Question

Web applications frameworks such as sinatra (ruby), play (scala), lift (scala) produces a web server listening to a specific port.

I know there are some reasons like

Answer 1

I would add:

• ssl handling
• for some servers like apache lots of modules (i.e. ntml/kerberos authentication)
• Web servers are much better for some things compared to your application, like serving static.