Quoted: what is the purpose of one?

There are two different 'standards' in front sight bases and removable carry handles.
The military spec parts are roughly 0.040" taller than the commercial parts.
These front sight bases are usually identified with an "F" stamp, signifying use with a "Flattop" receiver and removable carry hande.
Non-marked front sight bases are correct height for an A2 receiver or a commercial carry handle.
If you mix a milspec carry handle with a commercial front sight base, you will find that the front sight post is about 0.040" too short.
Hence the reason taller front sight posts exist.

Quoted: How the heck would one know if their carry handle was milspec?

By measuring from the flat on the bottom to the flat where the A2 rear sight would bottom out.
There is 0.032" difference.
I don't have the actual numbers handy, but it's likely found in one of the stickied posts.
Tweak will probably chime in before I bother to find it and link it...

Quoted: I don't have the actual numbers handy, but it's likely found in one of the stickied posts.

Here we go:
Text pasted from old posts I dug up.
Read it and try to understand it yourselves...

Damn, I had to measure my carry handles... 2 of them are Cardinal Forges and 1 is Anchor Harvey.
Other Cardinal came with Bushmaster Patrolman´s Carbine and the other came from Brownells and was sold as DPMS.
Anchor Harvey was also bought from Brownells as Prairie River Arms.

#1 Cardinal (Bushy) handle .7925"
#2 Cardinal (DPMS) handle .7935"
#3 Anchor Harvey handle .8325"

.8325" - .7935" = 0.039" (close enough).

Here are the measurements from four different Colt DCHs originally posted by Ekie in one of Tweaks threads. Measured the same way you measured yours. The measurements of all four Colt handles are .832 and yep, there is a difference.

My Bushmaster detachable handle measured .793" between the bottom surface that mates to the top of the rail and the top surface that the sight base bottoms down against. Measured at the very rear of the sight base, just behind it and behind the attachment clamp part.

The COLT carry handle rear sight base (shelf) measures .833", the Bushmaster .791"... There is a .042" difference between the two...

I measured 2 of my carry handles that are cardinal forge made for RRA and Bushy

RRA - .837"
Bushmaster - .797"

So it seems that RRA is mil-spec too.. yihaa

As Ive already posted here and in just about all the other different threads on this same topic...
Cardinal Forge makes two different carry handles, one is milspec and the other is commercial... The milspec DCH has a .042" taller sight base than the commercial DCH...

The Cardinal Forge milspec handle off of my COLT (with "F" stamped FSB) has a rear sight base that measures .833" and the Cardinal Forge DCH off of my Bushmaster has a rear sight base that measures .791"... So, without a doubt, there are at least two different spec carry handles being manufactured to be used on the AR-15 flat tops...

OK, I put away my trusty olé iron micrometers and broke out the digital jobby...

Both DCH's are Cardinal Forge and have different style RSB's...

Measuring to the middle of the windage screw gives similar results as measuring the DCH's "shelf" where the RSB mounts to, the main difference between the two handles is the height of the DCH where the RSB mounts to, not the RSB itself...

The following results were calculated using two Cardinal Forge DCH's...

Bushmaster: DCH base height = .7915"
difference of .042"
COLT: DCH base height = .8335"

Bushmaster: bottom of DCH to windage screw = 1.006"
difference of .045"
COLT: bottom of DCH to windage screw = 1.051"

Bushmaster: bottom of DCH to aperture "large hole" = 1.247"
difference of .045"
COLT: bottom of DCH to aperture "large hole" = 1.292"

Even though both RSB's are different type, they both are pretty close in spec @ +/- .003

And then, ta da...
The history lesson:

1982:
At Picatinny, Vince De Siena and Major Dave Lutz (USMC) machine off the carrying handle of a M16A1 upper receiver, affix a commercial Weaver scope rail, and then mount a Kahles 1.5x optic. MAJ Lutz begins shopping the idea around a possible addition to the list of features for the M16A1(PIP). (Lutz also believes that this prototype may have been the genesis of the later Canadian flat top project, due to his sharing an office with the Canadian Army Liaison Officer to JSSAP, Major Rick Wilson.)

August 1989:
Richard Swan of A.R.M.S., Inc. is shipped a sample of the Colt ACR's upper receiver and forging along with a purchase order for reengineering the upper receiver's scope rail. One of the main goals is to increase the strength the rail, as the existing rails cuts make the receiver too thin. (Reportedly, Swan demonstrated to Colt's Robert Roy that he could pierce the receiver at the bottom of the cut using the point of a Number 2 pencil.)

August 1990:
Colt and A.R.M.S., Inc. sign a non-disclosure agreement relating to their improved flat-top rail design. Oddly, the final design does not match the dimensions of Swan's earlier rail designed for the Canadians.

1992:
Colt commercially introduces their flat-top receiver for rifles and carbines. These are commercially designated the M16A3 and M4A1 respectively. (However, these weapon's features should not be confused with those of the military type-classified weapons using the same designation.)

August 1994:
The US Army officially adopts the M4 and M4A1 Carbines. Colt is awarded ~$11 million for 24,000 carbines. Only the first lot of M4 will be delivered with fixed carrying handles. Afterwards, all M4/M4A1 in inventory will be shipped with flat-top upper receivers.

February 1995:
MIL-STD-1913 is approved, providing a standard for accessory/scope rail dimensions.