What do I do with the renown that I get?  I dont really get this event and how to earn rewards.

We'll be releasing a guide on the main helpsite in the next few hours with all you need to know! :D

In the meantime, you can see what rewards are offered by talking to the quartermaster in your war camp.

it just seems like people standing around fixing a building that doesnt need fixing and getting small amounts of free exp

You gain renown while 'fixing' the building; you could also turn on PvP mode for improved renown gain in increments of 10 instead of 5. Alternatively one can gain renown by fighting either those at one of the sites or caravan convoy of the opposing faction, or along with the caravan, divining. You could also 'deconstruct' the opposing faction's buildings, that gives renown and corresponding construction xp as well.