Over long periods of time this is important because as the environment changes the organisms will be able to change with it. Population must maintain a specific size to support this.
Sex limits the potential for population growth rate.
For every 100 individuals in a sexual population, only 50 will be female and thus capable for bearing reproduction in next cycle. For every 100 individuals in an asexual population, all 100 will potentially contribute in the next generation to reproduction. Many of progeny will be fit for habitat, but many will also not be fit for habitat. Thus, further loss of growth potential due to mortality. Most mutations give rise to deleterious alleles Asexual species accumulate these.