When the Menagerie is motivated, its coin output is not only doubled, but it produces 10 random goods of one type from the building's age. The coin production is boosted by enthusiastic population and by boosts of certain great buildings. The building can be plundered if it isn't motivated.
The building also has a secondary visual effect. If the player builds it, then they will be able to spot Sacajawea wandering around the city with some friendly animals by her side.