They aren't *strictly* necessary. That looks like the forward-most one, which does help direct air into the intercooler and radiator, so I'd certainly prefer to have it there. But the reality is you're fine without it. Unless you're really pushing the car, in which case you're looking out for overheating anyway.
So it's your call. Spend the money or don't.
I don't know WTF they're talking about for "splash shields" vs "underguards". Just get the factory stuff. From the picture the panel behind this one is missing - it goes under the back half of the engine and the transmission. It has to be removed to drain the oil, so it might just be laying on the ground.