A Solo is similiar to a group, but adds the ability to toggle the rendering of nested object. It works like a radio button, so it will de-activate other objects on the same level. This is much faster than having to animate the opacity of each object indiviually.
One of the most common use cases for Solos is creating new skins for a character.
Sometimes when we animate an object or character, we need to create animations at different angles. Solos allow us to create multiple rigs and switch between them during our animation.
The ability to toggle between many images gives you a quick way to create frame by frame effects.