This is a question about the negative energy solutions to the free particle Dirac Equation in the first quantized picture. We need both the positive and negative energy solutions to have a complete set of states.
For example, the bound states of the hydrogen atom consists of superpositions of positive and negative energy free particle solutions. So then, what is the physical interpretation of a superposition of positive and negative energy states?
Suppose we had a hydrogen atom, and we immediately removed the nucleus. If we measured the energy of the resulting free electron, what would we see?