Thanks to Lisa for her explanation:
I used to work for the Ohio Ready Mixed Concrete Association, and briefly, it is inevitable that concrete will crack. You have to go back and saw a slab to put grooves in it so when it does crack, they will be hidden in the grooves. (Similar to perforations in a roll of stamps or indentations in a candy bar.) This is especially true in climates like Ohio and Indiana where the ground freezes and thaws (and so moves slightly as the frozen water expands) which forces the concrete to move along with it.
Now that's out of the way....I can't wait till summer! My daughter and I are looking forward to our annual girls' trip to Holiday World. Look for us on the carousel, Gobbler's Getaway, and seat 1-1 of The Voyage. (The trains look really spiffy, btw.)
And here's another explanation; this one is from Daniel in Cincinnati, who's exchanged occasional emails with us since he was a mere pup in engineering school:
Paula, why do you tempt me like this? You make it too easy.
Most structures made of concrete, especially thin ones such as a slab, will inevitably crack. There's no way to avoid it over large areas of slab. This can be due to a variety of causes, such as heavy loading, but the most common cause in a slab is fluctuation in temperature. The size of the cracks can be reduced by using appropriate amounts of steel reinforcing, but the cracks will still occur.
The workers in your photo are saw-cutting joints into the slab about an inch deep at regular intervals in order to cause the slab to crack at specific locations. This allows the slab to retain a more aesthetic appearance, as well as preventing random cracks at inappropriate locations.