Once normal cells are formed, they have a finite number of cell divisions they undergo during their life-span and, with few exceptions, they remain relatively localized to the same point in the body during this entire process. This allows the cells to determine if all around them is well and good and instructs them to maintain their relative positions in the body. Towards the end of a cell's designated life-span, certain cellular events occur which instruct the cell to terminate. This is called apoptosis or programmed cell death and is a very normal and useful event for clearing away the old and making room for the new. While it may not sound like the most compassionate thing to do to old cells, the evolutionarily derived utility of such action affords the body a mechanism to remove cells which might be on their way to becoming cancer cells via mutations.
All cells suffer mutations.
These mutations can come as a result of environmental agents, toxic products of metabolism, or arise spontaneously. The exact mechanism is not important. What is important, however, is that evolution has built into the cells mechanisms to deal with these. Apoptosis is one way to get rid of cells which have had ever increasing chances of suffering mutations that are irreparable. When, for various reasons, they cannot be, then generally apoptosis ultimately will make the mutations moot.