Certainly a degree can help! However, I'm not sure it helps as much as it used to simply because so many people have degrees nowadays. But that's for another discussion...!
The reason an interviewer will ask why you took a degree when you are being interviewed for a job that doesn't require a degree is that he or she wants to know what the thought processes were that led you into starting the degree, and to find out what you think university and having the degree has done and will do for you. We want to know whether you went to university because that was just the next logical step (ie no thought whatsoever), or because you had a particular aim in mind. We also want to know whether we are your second choice of career, which you're trying because your degree wasn't good enough for your first choice. Don't assume that we are necessarily impressed by the fact you have a degree; many complete plonkers have them! I'm sure you met some at uni...
Scroggs