Why are big slabs of concrete laid with tar in between them?

As temperature changes stuff will either contract or expand. If the parts are big enough there's a risk that one end will be of a slightly different temperature than the other end. That'll have one end trying to change in size while the other end wants to remain unchanged. This leads to tension in the material which can cause it to crack. By casting the concrete in divided sections the risk for temperature-induced cracking is reduced.

The space between slabs is filled with tar (or some other flexible yet water resistant mass) to take up the expansion/contraction of the slabs.