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Why have sex? To increase genetic variability (offspring get mix of parents' genes) which confers greater adaptability. To potentially provide two parents to raise offspring.
Why not have sex? Because it takes time and energy finding, fighting for, courting, and mating with another, not to mention the hazards. Because why mess with success (the genetic combination of the female worked, why not just clone her instead of taking chances experimenting with other genetic combinations?). Because every individual in the population gives birth to offspring instead of just half (females not males), the population can potentially grow faster.

What's the option? Parthenogenesis -- producing eggs asexually, without sex, through mitosis instead of meiosis. Instead of putting only half the chromosomes into the egg (and getting the other half from the sperm), duplicate them all! This results in a clone of the female (genetically the same except for copying errors). No longer is time and energy wasted looking for, fighting for, courting, and copulating with another, you haven't messed with success, and now everyone is producing offspring so you can recover from heavy mortality due to predation, weather, etc.
What's the downside of parthenogenesis? Low adaptability. Without genetic variation, when the climate changes or a new disease comes along, or the predators or parasites get too good at getting you, you have no options and go extinct. Also, two parents can raise more than twice as much offspring as a single parent, then sexual reproduction will be favored.

Solution. We see both strategies used in nature. During relatively stable times, parthenogenesis works, but sexual reproduction eventually prevails when environmental conditions change. Some species (e.g. aphids) produce both sexually-reproducing and parthenogenic offspring at different times of the year.