Right now all queries return objects from the root of the query (i.e. even if you are querying down through links, you are only able to use it to filter the set you started with). With a gather operator, you would be able to collect objects at bottom of the query instead.

An example would be if you wanted to get a list of all dogs owned by friends of yours:

auto friends_dogs = (people->column<bool>(is_friend) == true).gather(people->column<LinkList>(dogs))

This kind of operator is often needed for graph queries.