Why is it good for 2 parents to produce an offspring and have different genes?

Answer 1

Sexual reproduction that causes genetic mixing based on the genetics of both parents is a good thing in evolutionary terms, because it creates new genetic combinations, which creates more variation than you would get from asexual reproduction, and variation in turn allows for natural selection to operate. If every organism in a species is effectively the same, then the principle of survival of the fittest has very little to work with, since all organisms are equally fit. If there is variation, then the better variants will survive and reproduce more than the worse variants will, and as a result, the species will evolve, and become better adapted to its environmental niche, whatever that may be.

Answer 2

Two organisms are more desirable than one in reproduction because greater variety is achieved when you merge two organisms with different DNA. If a single organism reproduced the replica would be a exact copy of its parent. With two organisms combining their different DNAs, a unique organism is created, which may have certain attributes which makes it superior to its parents, and therefor have a stronger chance of survival. The exact replica dies, while the unique one survives.