It's part of the normal PCV system and they do it for emissions reasons. The other option is to put breathers on the valve covers and vent the crankcase to the atmosphere.
The one on the other side goes to the intake tube and doesn't pull as much vacuum as the one in the intake manifold so usually you just put one on the intake side. Forced induction engines sometimes go with one on each side or breathers.
As for being necessary that is up to you. I've seen forced induction engines detonate due to the oil in there. I've also worked on enough engines to see how nasty the inside of the intake gets with that oil making it's way in there or even gumming up the injectors. Granted that's over time but I'd rather have it on there and prevent some of that mess if possible even though I don't plan on doing anything to this engine.
Most engines have PCV type systems regardless of the fuel system.
I bought mine here http://www.billetdepot.com/ford-truc...tor-50-11.html