Antibody tests can help you understand if you've been previously exposed to the virus. For some, this is useful in answering that question—have I already had COVID-19? After testing positive for antibodies, you can even donate your plasma to studies looking for treatments.
Secondly, antibody tests may be a potentially valuable tool for public-health officials. They can help researchers understand how the virus spreads, how deadly it is, and how many people have come into contact with it. Plus, there is hope: according to infectious disease experts, it is possible that COVID-19 induces some degree of resistance against future infections.