But wouldn't this just serve to tell [them] with what to infect you?

You would know to what you are immune and they would know to what you are not.

NO MANDATES- Kansas Sheriff Uses World Class Antibody Test To Let Employees See If They’re Immune From COVID

https://nationalfile.com/no-mandates-kansas-sheriff-uses-world-class-antibody-test-to-let-employees-see-if-theyre-immune-from-covid/