The point of the contraction joints is to force any non-structural cracking, that usually occurs due to shrinkage as the water dries, into the pre-cut joint. You see these all the time on sidewalks for example. It's impossible to prevent shrinkage cracks and so convention say it's better to plan for them and get them to go hide in the pre-planned joint than to appear at random. These joints usually made by putting a small T-like piece on the face of the form. Or in a slab by inserting such a piece into the surface of the concrete.