Value of Owl Pellet to Science
Different species of owls often have very different diets, and the diets of individual species may vary considerably from place to place and season to season. Understanding any changes to the diet is critical for conservation efforts.
So, as you might imagine, owl pellets have proven to be an enormous boon to the scientists that study owls. Precisely because they contain such complete parts of the things that owls eat, they are easy to interpret, rendering a complete picture of the diet of the owls, as well as what prey species live in the area. There have even been recorded cases of the tags on smaller birds being found in owl pellets, providing information regarding their migrational patterns. As mentioned previously, because they are found beneath owl nesting and roosting sites, they're a good way to study their habits. They're also easy to come by, as an owl creates one after every meal, they don't degrade quickly, and they're a way to study owls that is noninvasive to their normal living habits.
This ease of interpretation also makes them an ideal introduction to science for young students, once the barf jokes have stopped being funny. For this reason, they are commonly sold to schools anywhere from elementary to college level to teach basic biology and ecology. (I was one of those third grade students, once upon a time!)
A warning, however: owl pellets are not sterile, and often carry the viruses and bacteria of the organism ingested, including some that are contagious to humans. If you find one and wish to dissect it yourself, sterilize it in a microwave oven beforehand, and use latex gloves while handling. There have been salmonella outbreaks in schools related to the improper handling of owl pellets!