As children we called them dirt dobbers but this insect is also known as the dirt dauber, mud dauber, dirt diver, mud wasp, and solitary wasp.
What purpose do they serve? Dirt daubers are a beneficial insect that we like to have around the garden. They are predators, and they love to eat spiders. Well, any creature that kills spiders is a friend of mine. (Sorry, I know spiders are beneficial in the garden, but I come from a long line of arachniphobics). And that is why I allow dirt dauber nests to remain in the garage.
Each species of dirt dauber has its own favorite prey. The blue dirt dauber is the main predator of the black widow spider.
Adult dirt daubers stock their nest with spiders to feed their offspring. They prefer particular types of spiders and certain sizes too. They capture the spider by stinging him, not to kill but to paralize. Rather than choosing a couple of large spiders, they cram their nest with a couple dozen small ones instead.
Dirt daubers are not aggessive and need not be feared since they rarely sting humans. They really should not be killed, but if the nest is in an unwanted place, you can use a putty knife to scrape it away. For more information on the dirt dauber, please visit Pollinator.com.