One major advantage of sexual reproduction is that it creates diverse offspring! Look around your classroom, and you will see that no two individuals are the same. Barring identical twins, no two people on earth have the same genetic code in their DNA. This is due to the unique combination of genes that occurs when human beings produce gametes and combine them in sexual reproduction. Diversity gives a population of individuals a huge evolutionary advantage because it increases the population's chances of survival given that their environment changes (which it always does at some point). For instance, let's say a virus hits a population of sexually-reproducing organisms. Some of those organisms may be susceptible to the virus and be killed by it. However, some individuals may have immune systems that are programmed differently due to differences in their genetic code, and these individuals may be able to survive an infection by the virus. This will allow them to survive and pass on their virus-resistant genes to the next generation, therefore creating a population of virus-resistant organisms that are more well-suited to survive.
Oppositely, asexually-reproducing organisms are all genetic clones of their parent (all have the same DNA). Therefore, a population of these organisms has very little diversity, if any at all. If a virus hits this population and it happens to be deadly to one individual, it will probably be deadly to ALL of the individuals, wiping out the population completely. Uniformity in asexually-reproducing populations is a huge evolutionary disadvantage.