WoWDB is used to power most of what's behind MMO-Champion's news. It replaced db.mmo-champion.com that was becoming super obsolete and wouldn't have survived MoP. It's also a way to try to improve user experience on MMO-C over db.mmo-champion.com pages we had because those were pretty bad.
It's a technological sandbox to let us improve the database platform on a company level. The improvements on wowdb are partially the reason why gw2db was more succesful than other databases for example.
The original reason why MMO-C had to create a DB was already mentioned in the thread, Wowhead got too big to be able to work with us and we were stuck with the choice of either doing our own thing, or posting really shitty news. The amount of data processing that goes into patch news requires way too much access than what we could have gotten from another company.
Some people also mentioned that competition is good, and I tend to agree with that. WoWDB was the first database to support a bunch of things such as spec variable spells or item upgrades. We also focus a lot more on newsing, so our spell parsing tends to be much cleaner to keep the unofficial notes clear:
http://www.wowhead.com/spell=100780/jab /
http://www.wowdb.com/spells/100780-jab
We also have random features like reagents breakdown
http://www.wowdb.com/spells/93328-vial-of-the-sands
The goal isn't really to compete with Wowhead, it's more of a pet project to see if we can find new ways to improve MMO-C and make the experience more enjoyable for users with what we have.