Many key events need to happen during the cell cycle to ensure proper cell division. For example, DNA replication must have occurred with minimal error (or at least without critical errors), cytoplasmic components must be segregated, the mitotic spindle must be properly formed so that the two daughter cells have the correct number of chromosomes.
As the cell divides normally, there are mechanisms in place to 'pause' cell division at checkpoints until these key events have occurred. This is one aspect of cell cycle regulation.
Also, each tissue needs to be maintained in its appropriate size and morphology. Thus, while cells need to divide to maintain tissue, they should also know when to stop. This is also done by cell cycle regulation.
In cancers, there is invariably some kind of mutation that breaks this system of checkpoints, thus allowing cell division to proceed even when things go wrong, or when the division is not supposed to occur, resulting in dysregulation of proliferation, and eventually tumours.