Are these necessary and if so what purpose do they serve? The sunroof has one built in that pops up when the sunroof is opened.

The built in one on the sunroof stops the thrumming sound. If you test drive one, try folding down the deflector and you will hear what I mean.

I installed a OEM Toyota hood deflector and I am disappointed with it. It will protect the front 2 ft of the hood from stones, but it won't deflect bugs or stones from the windshield. I got a stone star about half way up. When I took my 4Runner to Vegas, my windshield had lots of bugs. I looked at some other SUV's in parking lots and there didn't seem to be any difference in the ones with hood deflectors or without. I like the appearance, but it makes it harder to wash the hood. I'm debating on taking it off.

I don't even like the appearance. I think it looks like a bad mustache.. :)