My parents moved my sister into the basement when she grew up because it was basically an apartment. It was a lot bigger than her bedroom, and offered her the chance to accumulate some furniture and stuff that she would need when she moved out.

The basement is an upgrade. It's virtually a bachelor apartment (or a flat, for the Europeans reading), often with its own access, a private bathroom and often a kitchen or kitchenette. They're actually lux enough that they do very well on the rental market in busy cities.

For the child of the adult owners, it's a sort of independent-dependence. Some move down there while trying to save money for their own dwelling, which is probably the most often case, and others move down there, never strive for independence and become the stereotype talked about in this thread.