These are the first two boosters used to launch a Falcon Heavy. They are modified Falcon 9's. They already know much of the wear and tear that Falcon 9's endure

These are the first two boosters used to launch a Falcon Heavy. They are modified Falcon 9's. They already know much of the wear and tear that Falcon 9's endure in flight from previous launches. They didn't yet know how their boosters are affected by the modifications required, and the stresses involved in launching a Heavy. Now they have two pristine pieces of data they can tear down and analyse for potential improvements.