Hobbies And Interests
Sperm Storage
Unlike many other animals, a female black widow needs to mate only once. Females can store males' sperm and continue to fertilize their own eggs. Consequently, a female does not need the male after mating. Further, males have only a very slim chance of finding a mate after breeding with one female, so it may not be evolutionarily beneficial for them to continue living and competing with other males that have not yet mated.
Nutrition
As with all animals, it's important to the survival of the species that parents produce healthy offspring. Well-nourished spiders lay eggs that are more likely to produce healthy offspring. Eating the male may provide a female with the added nourishment the spider needs to lay as many healthy eggs as possible.
Effective Mating
Maydianne Andrade of the University of Toronto has found that, when females eat males, the mating period lasts for twice as long. Given that females can store sperm after mating, they may receive more sperm from this prolonged mating period and therefore be able to lay more fertilized eggs.
Predation
Female black widows instinctively bite anything that invades their web because they have poor eyesight. The male signals its intentions to mate by emitting unique mating vibrations. After mating, the male no longer emits these vibrations. As a result, the female may bite the male for the same instinctive and predatory reasons the spider bites anything else that enters its web.