A lot of old time bodybuilders would do the pullover in conjunction with the squat to get as much air into the lungs and along with the extreme stretch from the pullovers, the idea was that it would expand the rib cage.
Whether you can actually expand an adult ribcage noticeably is debateable and I myself am sceptical.
Having said that, I think the pullover is a great exercise and I use them regularly in my workout however, I feel that it's more of a lat exercise rather than chest since you're bringing your arms from over your head down towards and into your body, very similar to a straight arm lat pull down.