Each time your cell divides, an exact copy/duplicate is made which includes the DNA stored in the nucleus. That’s the genetic code that instructs your body on where everything goes – the blueprint that makes you who you are and that makes each cell in your body behave the specific way that it does. If that DNA is copied incorrectly, then that leads to problems with the function of the cells in your body which can in turn result in problems with your organs and other issues that can make you unwell or ultimately lead to death.
And when one cell goes wrong during mitosis (the technical term for the split) this inevitably leads to the spread of those errors – because the body doesn’t always know that the faulty cell has gone wrong so it will copy it in just the same way it copies healthy ones. This leads to the proliferation of that same error throughout your body which is how cancer spreads.