I can't answer with any certainty (since I am not in the access business), but I will give you my best guess, as I asked myself this same question..
1. They check for addresses that you've lived at, to verify what's on your PHQ.
2. They want to make sure you aren't in dire financial straits, as that would potentially make you susceptible to a financial bribe from an outside source.
3. General trustworthiness or reliability
With that being said, I had a number of issues on my credit report from a a few years back, and each of them required an explanation to the access personnel (no problem, it's understandable and I expected it). I also had an address on my credit report that wasn't on my PHQ that was questioned. It's an address that I have never lived at, and one that (after checking) doesn't even exist.. I verified this, with the access interviewer right next to me using google maps.. Just be upfront with everything. They'll find out eventually anyways.