Making sure trades settle properly, the correct accounts have been hit. Keeping up with trade confirms.
If you want to do it, only do it for a short while. It's a good learning experience for terminology and overall the nitty gritty of various securities markets. However, it is also my experience that it is the home of the most bitter people in the industry. And they get the short end of the stick in terms of perks and pay when compared to middle office (risk mgmt, financial controls) and front office (sales, trading) functions.
I think there's even a policy of even keeping them in a bad mood and bitter as we stash the back office employees at the head office of our firm in the sub-sub basement.