Evolution means the change in population over time. For this, you need to have a variation in a population. One way of having a variation is having mutations, but they are rare and a change of population based on the variation is very slow (the asexual evolution has to rely on variations).
one way to speed up evolution is mixing -up genes, and this is what the sexual reproduction does: mixes up the genes of two individuals to create another individual. This way with every generation there is a change in the genetic make up (the same is not true for the asexual evolution)