Sexual Reproduction is a process in which an organism has the potential to recombine its genetic material in order to produce offspring with differing traits from the parents. Evolutionarily, this can be very important in order to continue the lineage of a species.
You see, natural selection is heavily influenced by environmental pressures. It is the fluctuation of these pressures that facilitate nature’s way of choosing which organisms are most fit to survive in their environment. For example, having the genetic mutation that causes sickle cell anemia immunizes its carriers from contracting malaria. Sickle cell anemia is a genetic disorder found on chromosome 11 for the gene that makes the beta subunits of Hemoglobin. People who have this disorder had a mutation occur where the amino acid glutamic acid that normally leads to the formation of normal beta hemoglobin formation, has a point mutation in the DNA to code for the amino acid Valine instead. This causes the red blood cells to shape into a sickle instead. Normally, one would think that having this disorder would make a person less fit, however, studies have shown that sickle cell anemia has flourished in places like West Africa where misquote populations carrying malaria is the highest. This is because people who carry one copy of the gene mutation for sickle cell anemia or have sickle anemia are immune to malaria. Therefore, it is advantageous in places like West Africa to have sickle cell anemia and nature selects for them to survive over others who are not immune. So, if the people who are surviving in these areas are the ones who carry this genetic mutation they will be the ones more likely to survive into adulthood and sexually reproduce and sire children with this same mutation. Further selecting for the spread of sickle cell anemia in the population.
In summary, sexual reproduction shuffles up the genetic possibilities offspring can have from their parents (this process in called genetic recombination). Then nature selects for which combination of genes is most fit for the organism to survive/thrive in its current environment. In return, that organism is most likely to mate more frequently and push out more of it genes into the population gene pool than others who have genomes that are less fit. Without this, populations could die out if all organisms in the population carry the same genome as we see propagated with asexual reproduction.