We’re getting there! Your question is one of the great mysteries in biology, you know.
Another way to think of your question, in terms of evolutionary biology, is as follows:
Why is it more advantageous for an organism to reproduce sexually, rather than asexually?
Right? Because obviously, if you are a bacteria, or a jellyfish, or whatever and you split in two, and each of those two split in two—you get a lot of offspring very fast. Whereas if you have to monkey around with sexual reproduction, you don’t get as many offspring.
One way to answer this is to simply point out that most of the time sexual reproduction isn’t advantageous. The vast, vast majority of organisms on this planet are bacteria, and they reproduce asexually. Many simple animals like jellyfish often prefer asexual reproduction and only occasionally reproduce sexually.
So, you are basically asking “what tipped the balance towards sexual reproduction for that tiny sliver of life that now reproduces that way, the eukaryotes?”
So let’s look at how sexual reproduction—or rather, something like sexual reproduction (because it would have evolved gradually) pays off. Let’s say you are a simple bacterium. You reproduce asexually and form a huge colony of clones, just like all the other bacteria.
Now let’s say a virus infects your colony. Since you’re all clones, with the same DNA, the virus can easily wipe out your colony.
On the other hand, the virus doesn’t affect other species of bacteria. Their code is too different. So if you want to survive, bacterium, your best bet is to steal some of that other bacterium’s virus-immune DNA!
With me so far?