In cold weather areas the foundation footings need to be approx 4 feet deep to avoid frost heave. At that point it is very economical to extend those footings deeper and make a wall.
The cost per area is low compared to a fully finished space above ground. When I built our house I insulated the walls and floor of the basement and added pex tubing in the concrete floor for radiant heat. Cost was about 20% of the total budget and it doubled the interior space.
The added space is also ideal for all the house's mechanical systems and plumbing.
