As far as I know, Rack just sits between Rails and a web server (like Mongrel or Webrick) to act as an βadapterβ, so choosing a web server is not important.
So why is Devise known to work in a rack, and is that good? If Rack is transparent, doesn't it matter if the authentication system is based on Rack or not on a rack? (or how does this relate to Rack in general?)
It just means that the gem does not depend on the particular implementation (for example, Rails) of the Rack interface, just to match it. This is good because you can use the gem with other angles such as Sinatra.