Answer
With external fertilization, male parents can know with more certainty whether or not the offspring are their own.
Work Step by Step
Animals in general do not want to contribute to the survival of offspring that are not their own. In some animals with internal fertilization, there are significant adaptations in the males that aim to prevent the females from mating with other males. In some species this is behavioral, but in others the structure of the sex organs also heavily contribute.
With external fertilization, male parents can know with more certainty whether they are in fact the fathers of the offspring that they help raise.
