I think that this has been somewhat answered, but to give an answer myself: it can model time, just as does an individual action-point system. However, where individual action points model the time available to each unit, pooled action points model the time available to the commander: he can send out only so many orders at once.

To butt in, the commander is the player, and this gives a very convenient way of keeping the length of the turns within some rough boundaries as the game progresses,

(compared to civilisation- and 4X-games where at the end of the game you had to do a million things every turn.)

per turn instead of per unit?

I believe Space Hulk has an Hybrid system that allows you pooled APs on your units (based on a die roll).

Some games also allow you to activate only some of your units (not the entire list).

Personally, I think the per unit approach is more fertile for tactical placement: every unit needs to contribute if you are to win, because if you only focus on your big guns, someone will outwit you with his lesser units by making unexpected moves, or using them as meatshields to tank on the damage while they flank you with their bigger guys.

It demonstrates more skill and I'm a very competitive player :)

I like it. Here's my take:

You're commanding an army during the age of semaphore. No radio yet, but nor do you have to send runners with orders. So you can give orders to distant squads but it still takes some time. Your assistants at HQ have to encode the messages, squads have to decode them... compared to earlier methods it seems instantaneous but there's still a practical limit of how many messages can be sent. (There's also a visual limit to the signals, so to move the front you have to also capture enemy semaphore towers and protect your own.)

Your per-turn action point unit is expressed in terms of words: your assistants can encode 25 words of orders per turn. So you can say something like "move north north, attack west". (You don't necessarily have to type this in; maybe the game infers it from where you click.) Meanwhile, each squad can only decode 8 words per turn. (This is to prevent you spending all 25 words on your Thunder God Cid unit.)

As the game progresses you get a more nuanced vocabulary, and thus can achieve more per turn than you could with only simple verbs. "Patrol" is like "move" but the path specified gets repeated until new orders are received; "reinforce" sends the unit towards another unit without having to specify the path; "retreat" allows several spaces of movement per direction specified but only away from the front, etc.