Take a look at "Who Built the Pyramids? Now We Know!" How was the discovery of some tombs important to archaeologists' understanding of ancient Egypt?

The discovery of the tombs was important because it made it clear that the people who built the pyramids were paid laborers, not slaves.

The reasons they know they were workers include that they wouldn't have been given burial so close to the kings if they were slaves, and the material the tombs were made of (mud brick) was commonly used for the tombs of paid workers.