Why does Darius use mana?

I thought mana was supposed to punish bursty playstyles for being careless with their attacks? You get a ton of power over the curve but you only get a few tries. Everything about Darius is a sustained battle of attrition similar to Garen.

It just feels weird on Darius, y'know?

http://i.imgur.com/TM5Tq2P.png

Edit: So Meddler very kindly chimed in:

We put mana on champions like Darius primarily to add some cost and decision making to their ability use in lane (reduce 'just use it whenever it's off CD as a playstyle). Darius's costs don't scale as much as his mana pool does, so as the game progresses he'll be less and less limited by mana costs, with his gameplay shifting to be much more about target choice and positioning.

The only ability that has enough mana tax slapped on to contemplate mana management early game is your dunk. The things that restrict Darius' combo early are the same things that restrict it late -> positioning, except early he has no DMP, boots or ZZrot, no CDR so he can't spam anything even if you wanted to and his E is both his initiator and defense, burning this can leave you more vulnerable than most because it gives the lane opponent a huge window of time to have their way with you.

You'd have to increase his mana costs to dunk levels before it could contend with any of the factors that have a large impact on his actual decision making, which would require you to lower CD's and then he could actually spam **and that would be oppressive and very unfun to get E'd back to back even if he could only do it once. ** So Darius always has to be limited by CD's, which along side his immobility are his tuning knobs, not his mana.

His mana doesn't do anything and if it did his CD's or damage would have to be buff'd to compensate which would create a monster. Where as removing his mana doesn't impact him.