Sometimes additives may be used to accomplish certain objectives such as workability, etc. For concrete to prove durable, all components must be in a proper balance. Too much or too little of any component can cause problems later. For example, let's look at the water component. But too much water can result in excessive shrinkage and weakened concrete.
All concrete mixes contain more water than is actually needed to hydrate the cement.
This excess water eventually evaporates. One primary objective is to retain the water until the cement is fully hydrated, then let it evaporate slowly over a prolonged period of time. If concrete were left as it is placed, it would crack in random patterns. To avoid random cracking we create joints at regularly spaced intervals, thus weakening the slab in a grid pattern. Thus, a control joint is actually a designed crack.