because you don't know what they are going to ask, and all it takes is one slip. People forget things, like myself. I have very bad memory and for my line of work (programming), I have to constantly look up things on the internet as I can't remember everything.
There are thousands of things related to a couple for example simple things like parents' names, DOB, anniversaries... Not everyone can remember every single details like those. I remember in the ROC interview, the CO looked at the bank statement, pointed at one transaction that show Austin on it (we live in Houston) and asked what did you guys do in Austin. Totally forgot what that was, it took me a while to remember. I renewed my wife ID and Austin is where the payment was originated. So there's that. He also ask my spouse about my parents' names and she couldn't answer as she totally forgot about it. She actually never saw them in person as my parents still live in Vietnam, she doesn't interact much with them, how can she even remember their names? Things worked out ok, I got the conditional PR card, but what if the CO is ugly and tries to press it further?