The main reason is to make sure you are not a waste of time and the company is not wasting your time. I always talk about that basics of what the job is, the company culture and and what the company does. I talk about the location, schedule and the pay range too because if those are not in the realm for the candidate than why the heck would either side continue. I usually throw in some basic questions about their experience and comment on a few resume items.

Things I listen for:

Do they sound professional, competent and interested?

Do they know anything about the company already? Not knowing anything is not a deal breaker but it tells me they threw their resume at the job rather than made a calculated application. Knowledge about us is also not a deal clincher but it adds major intrigue.

Are they asking good questions and seeing between the lines on what the job is and what we are looking for? Can be a major plus. I have found the many of our best hires did this on the phone. When this happens I frequently ear mark that individual for other management who will be in the interview with me. I say- this person had great questions and sounded really intriguing on the phone so I'm very interested to see how they are in person.