Why use FPM ?
FPM IS FastCgi ! “F.P.M.” stands for “FastCGI Process Manager”
CGI is really slow. FastCGI, as its name indicates… Is not slow. CGI
is slow for the simple reason you have to completely launch PHP and all
its libraries & etc. … whenever FastCGI launches “hanging” PHP
processes that are waiting for requests to spend their time doing
something more useful than “hanging” in memory 
But this is the very fact why there is currently a problem with running
many websites with FastCGI : it’s memory consuming if you have so many hanging processes waiting for queries (as a php process is ~ 15MB, and even more when parsing a wordpress website ! (~35MB !) ).
(note : “hanging” here does not mean the unix “hang” signal, huh, it
just means hanging as anything in real life)
