We have boards containing cards from multiple activities. If we 'group by' activity, we can clearly see which cards link to which activities, which works great.
However if we want to 'group by' label or assignee, all cards for all activities are listed under each label heading, with no reference on each individual card to the activity to which it is assigned. This makes it impossible to understand the meaning of many cards when using these views.
A secondary filter would essentially allow you to 'filter by' activity after 'group by' is used to sort by either label or assignee.