Why does the Army uniform still have these giant cargo pockets? Who uses their cargo pockets, and for what reason?

It's called the ACU (Army Combat Uniform), not the AGU (Army Garrison Uniform). Personally, I carried my analog brain in my cargo pocket constantly when I was in garrison.

The cargo pockets (as well as the other pockets) serve a functional role to carry any number of items that a Soldier will need in the field or combat which needs to be readily accessible - MREs and other equipment (cargo pocket), extra magazines (lower leg pockets), etc.

I suppose the military could create yet another uniform - a "garrison combat" uniform that puts form over function and the current one that puts function over form.

I was in a Long-range Surveillance Unit. We used our cargo pockets all the time in the woods. A Soldier can fit a main meal in one, and keep it from freezing. a mag can fit in it, as can a watch cap or gloves.
As an Infantryman and a Paratrooper, I used the cargo pocket or maps as a Jumpmaster, for gloves or headgear as a Grunt...
They are useful in the field but a pain in Garrison

Snacks and tobacco. Can't have a war without em'.

I'm guessing you never had to go across the pond. yes, useful when in deployment situations but I agree, we should go back to a class B uniform for garrison ie khakis or the like.

MREs, magazines (the kind with bullets in them), note pad, work gloves, candy, cell phone, radio, hat, rag, Gatorade, towel, etc. It's a combat uniform.

I am going to go out on a limb and guess that the original poster of this question spent his career in garrison and very little time in the field. Just an observation as others have adequately explained the "cargo" that often ends up in the cargo pockets.

I put my child in there for being your kid to work day

As a grunt I used it to carry My soft headgear when wearing my kevlar, Mre, gloves, maps, mags, or anything I deemed that I might need quick access too while on a mission. Those large pockets are very handy. No new uniforms for garrison needed, an iron, elbow grease and some starch always got my bdu's to look good in garrison. The wasted money on new uniforms for garrison would be better spent on improvements on taking care of the soldiers in garrison or the field.

Like most troops, I threw my canteen in there as a private before we were issued our web belt. Now I use it to hold my little green notebook. Unfortunately its not as little as the pocket so it sticks out the top. I use to also carry some extra socks, MRE extras, extra magazines, paper. As long as it didn't jab into my leg while I was walking, it was fair game.

In Iraq I stuffed the cargo pockets full of extra magazines. Ain't catchin me running out of bullets.