Because of the cost to the female of carrying the foetus and then expending energy in raising the offspring she will want a male partner of good genetic stock so that her offspring are healthy. It would not pay to invest all of that time and energy in an offspring which would not have much chance of surviving and passing on it genes to its own offspring. Animals which did not select in that manner would not survive themselves. Females cannot see or know what the genes are but appearance gives an indicator. A vigorous, large, strong, dominant male is preferred by the females of many species.
Elephant seals are a good case in point.
Male elephant seals are much larger than female elephant seals. The difference in size between males and females of a species is called sexual dimorphism. A male elephant seal might weigh four times as much as a female-perhaps 4,000 pounds to the females 1000. Male elephant seals guard their harems jealously. If another elephant seal tries to mate with a female there is often a ferocious battle as can be seen in the picture.