The mana pool is used in Manifest Destiny to moderate the use of magic, be they items, attachments, or spells. Items refer to any magical item that you bond to, that item once bonded uses the player as a battery to provide power for the items effects. This also applies to golemware, also called cyberware in other games. The golemware draws power from the characters mana pool to power it's functions.
The dice that an item or a piece of golemware use are simply removed from the dice pool like they were not there at all. If you unbond from the item or have the golemware removed, then those dice are returned to your dice pool. So the Mana Pool acts as a way to keep players from going over board. After all if you do not have the dice, you can not use the item.
The man pool is also used for casting spells, something that any character can do. Spells have two parts when it comes to mana, the cost and the pump. The cost is an amount of dice that are payed to form the spell. These dice are not rolled. The pump consists of any dice the player wants to add in, these are the dice that are rolled by the player and there is no limit. So if a player wants to roll twenty four dice, then they can as long as they have them.
So why use mana pool dice for the roll and not an attribute? Because your not rolling to see if the casting succeeds. Instead your testing to see the potential success of the spell reaching the target. Casting a spell is easy, but if it does not have enough focus and power...it will just evaporate. This is what makes the mana pool so essential, since you have to balance magical items, golemware enhancements, and your spell casting against one pool of dice.
I hope that you have found this interesting. I never really liked how some games used static attributes to determine how the different things interacted. And I really believe the player should have choice when it comes to what they can and can not do. If they choose to be a brawler, then magical items can be their way to go. While a caster would be item light. It is about choice, after all it is your destiny.