Cancer can be understood as evolution within the body. Cancer is a mutation, and that mutation is selected for, while the body is alive. The cancer cells are more "successful" than other cells, because they are selfish reproducers. They out-compete other cells for the resources of the body.

The coherence of the body depends on its cells retaining the information and the purpose that they inherited from the zygote. Over time, that information and purpose is degraded by entropy and changed by evolution. Without death, the body would eventually dissolve into a battleground of competing cells — a tragedy of the commons.

So, death is a consequence of entropy and evolution. The dynamics of sexual reproduction force the cells of the body to work together, rather than competing against each other (cancer). This is arguably the most important reason why sexual reproduction evolved. Sex, death, and complexity are all linked.