This is because the remote span VLAN is used to transport the mirrored traffic from the source switch to the destination switch, where the monitoring server is connected. The remote span VLAN must be defined on both switches and must not be used for any other purpose. The source of this answer is the Cisco ENCOR v1.1 course, module 6, lesson 6.2: Implementing SPAN, RSPAN, and ERSPAN.