Which is a valid reason to implement a wireless LAN controller?

Centralized authentication of clients is a valid reason to implement a WLC. Although a WLC makes it easier to implement multiple SSIDs and VLANs, this task can be performed with autonomous WAPs, each performing its own authentication. The use of autonomous WAPs negates the reasons you would use a WLC because each WAP would be independently managed and no coordination would exist between the autonomous WAPs. The use of multiple SSIDs can be achieved with an autonomous WAP without a WLC. Multiple VLANs can also be used with an autonomous WAP without a WLC.