'...it turns out there's a good reason for the lines. Not that cement contractors necessarily know what it is, of course. Many of them have the idea that the lines allow the concrete to expand and contract with changes in temperature - not true, strictly speaking.
Concrete does expand and contract, and for that reason expansion joints, typically some sort of compressible fiber board, are put in every 40 feet or so. But the lines you're talking about, which are called "contraction joints," serve another, admittedly related, purpose. Concrete normally shrinks a bit as it dries, resulting in unsightly cracks. Cement finishers put in contraction joints so that when the concrete does crack, it'll do so at the joints, where the slab is thinnest, rather than just any old place. That way it won't look so bad. Of course, a lot of times it cracks any old place regardless of what the cement finishers do, but hey, they tried.'