Priority Group Activation, why use it?

If I may add, ratio and priority group are different things. If all or some of the members of a pool have the same priority, then any ratio differences will be applied. If a pool member is in a higher priority group than other members, then ONLY that pool member will be selected, until the PGA threshold is met. So for example:

1.1.1.1    Ratio 1    PG 4
1.1.1.2    Ratio 2    PG 3
1.1.1.3    Ratio 1    PG 3
1.1.1.4    Ratio 1    PG 2
1.1.1.5    Ratio 1    PG 1
1.1.1.6    Ratio 1    PG 1

As long as the PGA threshold for PG 4 is not met, only that member will be used. Once it fails, as long as the PGA threshold for PG 3 is not met, only members in the PG 3 group will be used. They will, however, use a 2:1 ratio for load balancing. Members in PG 2 and 1 won't get used until the higher level groups are exhausted respectively.