Most crabapples are planted mainly for the ornamental appearance of their blossoms and fruit. Depending on the variety, the fruit may hang on the tree into the fall and winter, and be left to feed the birds. One drawback to letting birds snack is that some nonnative ornamental crabapples have become invasive due to the birds spreading the seeds. In addition to their ornamental value, some crabapples serve as pollinators for regular apples.
Your crabapple may be helping the apples in your yard bear fruit. Crabapples will continue to bear without being picked, though they may produce a heavy crop one year and little or no fruit the next. This tendency varies with variety. Tart crabapples can add body and character to apple cider. Otherwise, try making jelly or pickled crab-apples out of them. Crabapples are useful in making jelly because they contain so much natural pectin that it often isn't necessary to add commercial pectin to make the jelly set.