Why does the slave dealer change Northup's name? Why would it be common practice to rename slaves after their owners?

The slave dealer named Theophilus Freeman changed Northup's name to Platt to take away his identity. Losing his name made Northup realize that he has lost his connection to his family and he has lost his freedom and became a slave.

Slaves were sometimes named after their owners to prevent confusion. For example: A master has a slave named Sam. Then he bought a new slave also named Sam. The new slave is then given the surname of his previous owner which might be Butler so the new slave's name became Sam Butler.