Why does cell division remain important to an adult organism even after it is fully developed?

Cell division prevents cancers, and is something that in humans and in other adult organisms as well prevents old age.

There are many more complex mechanisms that contribute to old age and cancers, but if cell division slows down an organism reaches the end of its life.

Every set of cells has a limited amount of divisions it can do. literally every organism from the moment of conception is dying.

it is very expensive

explanation:

unicellular organisms, like bacteria, are able to perform all life functions within one single cell. multicellular organisms need many different types of cells to carry out the same life processes. each of these special types of cells has a different structure that it perform a specific function.