My understanding of Cluster groups is different (I may be wrong, it won't be the first time, just don't tell my wife). Let's say that you have 4 cluster members MC1, MC2, MC3, and MC4. And let's say that MC1 and MC2 are in the West closet and MC3 and MC4 are in the East Closet.
West. East
MC1 MC2. MC3 MC4
When the cluster leader assigns the AP anchor controller (AAC) and Standby AP anchor controller (S-AAC), it is possible for the AAC and S-AAC to point to MC1 or MC2. This logic goes for the User anchor controller (UAC) and Standby User anchor controller (S-UAC).
If the AAC and S-AAC are pointing to one closet, and if that closet goes down, you will have a problem, as the AP will not be able to fail over to it's S-AAC, since both the AAC and S-AAC MCs are down. Again, this would be a similar problem with the users as the UAC and S-UAC would be in the same closet, and both the UAC and S-UAC MCs are down.
By puttting MC1 and MC2 in one cluster group (which is literally just assigning a group number to the two MCs during the definition of the cluster), and putting MC3 and MC4 in a different cluster group, when the AAC and S-AAC are assigned to MCs, these will be assigned to controllers in different cluster groups. (Again, same concept for assigning UAC and S-UAC). In this situation, if one of the closets goes down, any AAC or UAC assignements in that closet, will have their standby assignments (S-AAC or S-UAC) assigned to a controller in a different group, which are the MCs in the other closet which is still up and running.
I hope this helps,