A nice quote from Razib Khan…
Mutations are as you know a double-edged sword. On the one hand mutations are the stuff of evolution [...] On the other hand mutations also tend to cause problems. In fact, mutations which are deleterious far outnumber those which are positive. It is much easier to break complex systems which are near a fitness optimum than it is to improve upon them through random chance.
[I]n a polygynous population a few healthy males with good genes could give rise to most of the next generation, and so providing the balance of selection to the background mutational rate. [While the rest of the males serve as a "dumping grounds of bad genes".]
In contrast monogamous populations will have less power to expunge mutations in this fashion because there is more genetic equality across males, the bad will reproduce along with the good, more or less.