This depends on the topologies which are being being used for the wireless networks.
When using bridged topologies, then session availability will take care of the roaming of the clients to an APs which is local to the other controller.
As the client stays within the same subnet it can keep its IP address and the mobility feature is not needed.
When using routed wireless topologies, then the wireless subnets for the same SSID will most likely be different on the two controllers.
Without mobility enabled, then the client will renew it's IP address when roaming to an AP that's local to the second controller and this will interrupt client communication shortly.
To avoid this, the mobility feature should be configured between these controllers, as then the client will keep his original IP address and traffic will not be interrupted.