The JVM settings are used in the hierarchy domain->S-group->host-default->server.
The values are inherited and you might override it.
Normally if you have a server-group it contains normally i.e. a cluster which share the same application and load. In this case you need to have the same amount of memory.
For other cases you might have unsymetric load or the hosts are not equal and you need a different JVM setting.
Also you might enable some monitor options at server level.
Does that makes sense to you?