Drop Broadcast and multicast drops the broadcasts and multicast traffic at the controller. A broadcast is unicast from the client to the controller, where it is forwarded to the wired network, but NOT back into the wireless network. Your second VAP allows this traffic to be put back out onto the wireless network, so your clients will send, and see broadcasts on that subnet. If enabled, this knob will drop all broadcasts and multicasts on a VAP except DHCP.
This helps to eliminate unnecessary Broadcast and Multicast traffic over the air which in turn increases the performance. Thus it is helpful in optimizing the performance. However if we use any application that uses multicast traffic, enabling this option would affect it. Hence if clients do not require multicast services then enable this knob.