Finally ordered an upper for the stripped lower sitting collecting dust.  I ordered an M4 style upper with a flat top receiver...  

My question is: what purpose does a LaRue rail serve?  The flat top receiver already has a picatinny rail.  Is the LaRue solely for a QD of optics and raising the optic 5/8" higher?  

Sounds like you are talking about the Larue riser LT101.  The purpose is to create a QR scope mount.  If you have an FSB and flip up rear sight, you will need to raise the line of sight ~1.6 - 1.7 above the rail.  Without a front sight in the way, 1.4 or 1.5 is probably ok.  This depends on BUIS, no BUIS, cheekweld, etc.  For me, I got my Millet DMS-1 to 1.7 with a LT101 and TPS medium rings.  I need it this high to clear my BUIS and FSB.  1.7 still gives good cheek weld for me.  It also moves the scope away from your eye.  This is important when prone, especially.

Another choice would be super tall rings or dealing with a low scope.  I can't imagine where you will find 1.5" rings, but I think someone makes them.

Other scout rails are available from GGG, Millet, ARMS, etc, but Larue is the best.

This is exactly what I was looking for.  Thanks!

So, if I don't plan on having iron sights at all, the sole purpose would be for QD of the scope?

Larue rails in free float allows barrels to breathe and not be affected of crap tugging on them screwing your accuracy, allowing you to use a PWS gas piston system to keep your rig running clean and accurate.  As well as holding cool gear in a great way and looking "kickass" while doing it.  
And thats just for starters.

Quoted: This is exactly what I was looking for.  Thanks! So, if I don't plan on having iron sights at all, the sole purpose would be for QD of the scope?

Well, QD is not exactly the sole purpose.  Along with raising the optic just a bit for cheekweld/eyeball line-up, I can think of these reasons:

- The extra height let you use an optic with a bigger objective lens, without that lens coming in contact with your handguards out front.  (bigger obj. lens = bigger FOV and light gathering)  

- The extra bit of rail length out front lets you mount the optic further forward than the flattop alone would allow, so you can move it out there and get the best eye relief and cheek weld. Incorrect eye relief sign....

Both of these points give you the ability to use a wider variety of traditional optics on your AR-15.  Also note that LaRue has some mounts that already have rings built in, not just a riser-rail like the LT-101.  To be honest with you though, I like my "101" just fine as 1) shooting is not my occupation, 2) It lets me use a set of rings I already had laying around, and 3) It didn't break the bank.

I'm sure there are lots of reasons I'm forgetting, but that's what comes to mind offhand.

Tex78

Edit to add:  Dont forget to check out the "AR15 optics" forum HERE as you can prolly find some pretty good info on your upcoming purchase.  Also, check out the "picture" thread tacked to the top of this board for a lot of ideas on settups, questions to ask etc.  It really helped me get some idea's together on my last build.  And BTW, welcome to the board.  
-Tex78