Nobody owns that account - it has no keys as you can see here: https://hiveblocks.com/@steem.dao
It gets funded 10% of the reward pool every day, as you said. There was an initial donation from Steemit, Inc. of 200,000 STEEM soon after the DAO was created. The majority of the funds came after the launch of the Hive blockchain, when the ninja-mined and promised for ecosystem development Steemit, Inc. stake was sent to the fund. Also, anyone can donate to the fund or set it as a beneficiary to receive post rewards.
The paying itself that you saw might be the unallocated daily amount which goes back to the fund. In other words, there is a cap on the funds that can be paid out per day. If the actual amount paid is lower, the rest goes back into the fund.
What other purposes will this account used for?
Hmm. I guess this is up to us, the community. Any other uses come to mind?
Note that the account will be renamed to a Hive-specific name in a future hardfork (I'm not exactly sure if it will be in the next one).
Ok great thanks for the info! Yep I had assumed a bulk of that stake likely came from the original ninja-mined coins. It is awesome to see that get allocated to where it will actually be used!