There is quite a few reasons in terms of pros and cons when configuring controller redundancy with 2 controllers.
Master/Master Backup = Uses VRRP and will only work in a L2 domain.
Master/Local = Will work in a L2/L3 domain but AP's will rebootstrap to establish tunnels.
HA Group = Works over L3, GRE tunnels already established so fastest fail over.
Take a look at the Campus WLAN Redundancy VRD (2017) for further information.