To support hiding parts of rigs (i.e. entire objects), the Spring team needs to be able to hide some of the objects that make up the rig. For instance, things like the corneas, the scarf, etc. (i.e. see the custom properties on the master bone). However, to do so, they need to be able to set drivers on some kind of visibility setting.
It was concluded here that the best solution was to make collections fill this role, so to support driving the visibility, collections need to get animdata so that drivers can be added to the setting.
(In future, this may also allow animators to have animation for multiple characters defined in a single action, as long as both characters belong to the same collection. Some work would be needed with custom keyingsets to make that workflow work, but it is something this change would also enable, as a side effect of collections having a list of member objects)
See the tmp-CollectionsAnim branch