This is because the cells we need for reproduction need to be genetically different from those of the rest of the body.
Cellular reproduction
When a mother cell divides into two or more daughter cells, this process is known as cell division. Cell division frequently takes place as a component of a longer cell cycle. There are two different types of cell division in eukaryotes: a vegetative division (mitosis), in which each daughter cell inherits the genetic makeup of the parent cell, and a reproductive division (gametogenesis), in which the number of chromosomes in the daughter cells is cut in half to produce haploid gametes (meiosis). In cell biology, the process of mitosis, during which replicated chromosomes are split into two new nuclei, is a stage of the cell cycle. The number of chromosomes is maintained in the genetically identical cells produced by celldivision.
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Answer: The literal reason would be because our ancestors did. The ancestors of all modern mammals had five digits on each limb up to 360 million years ago.
