What's the use
 

I've always wondered what the rear spoiler thing on blazers are for, does it actually function as something?

It was supposed to keep the back window cleaner (seriously).

What that's crazy, does it direct air away from it or something?

It directs air down over the window to keep dust/etc from swirling up onto the window from the road. It was one of the first things I added to my Rainier after I bought it.

Has anyone on here a actually tried with and without it and noticed a difference?

Many people have removed theirs to clean up the looks at the back.

On my Rainier, there was a HUGE difference in how clean the back end of the truck was.

I had one on my old Bravada as well, but that was on there when I bought it. The back end of that truck stayed a lot cleaner than other 2nd gens in the area as well.

It does make a difference. I had an '87 without it, and it was a lot dirtier than those with it. I often wished mine had it. For that matter, I wish my '95 had one.

I wonder why I don't see these in modern SUVs then, my moms Acura mdx could really use one, that back always gets real dirty

I've always wondered about this - my Dad had several blazer's as leases during the late 80's and early 90's - now I know. Thanks for the heads up. And I agree, I could use one on my '02.

Yeah I had one on my 92 and I never liked the look of it. When my back glass got busted I replaced the hinges also and went wingless. And now the water pretty much sits on my back glass when it rains. It stays dirty and is almost impossible to see out of when it rains, espically where my tires throw up alot of mist behind the vehicle and it coats the back glass...