How does better sperm result in more viable offspring?
Greater choice and variety of mates increases a female's chance to produce more viable offspring.


The claim is a little hard to address because it is vague. However, one can give a simple example to give you a feeling of why this could be the case.

This example is based on Zaahavi handicap principle. Spermatozoids aren't free to produce. A male that can produce more and stronger spermatozoids can do so because he is a healthy males. As many of the traits that determine health, in a general sense are heritable, healthier males tend to have healthier offspring.


  On the other hand, what helps a particular entity to survive is determined by many parameters. Some of them are: speed, endurance, strength, agility, stealthiness and savvy.


You are mainly listing traits that are very specific to human's sport ability. In reality, there are loads of other traits that are much more fundamental such as the efficiency of the potassium pump or of the mechanism to produce ATP.


  [..] it helps in natural selection [..]


I don't understand what you mean by this sentence.