This is a great read on the history and role of FastCGI given the rise and popularity of "CGI Frameworks", as the author calls them, like Rails, Django, and Zope. One of those rare entries where I learn a lot from each paragraph.
Why does this matter? Because these are large frameworks written in interpreted languages and using CGI (the other technology still supported everywhere that lets you tell the web server to run some code) you have to load the whole shebang from scratch ON EVERY REQUEST. Performance will be shit, trust me, and so for anything but development with these megaframeworks CGI is completely unfeasable. You need a way of getting all that code loaded into memory and have it stay persistent across requests. Thats what you get with an Apache module, and thats what you get with FastCGI. The renewed interest in FastCGI is because suddenly PHPs assumed rule of the webdev roost has been called into question and now you have these compelling competing frameworks, written in competing languages, needing persistance on the web server and theyre not going to get it with an Apache module.