Why did people traditionally do spring cleaning? Dustmites or toxins from heaters?

Ask them. I would imagine a lot of dust, household chemicals etc can build up all winter while the doors and windows are closed. Opening the windows will vent all of that. Dust mites are a little trickier to get rid of. But of course open windows bring new dust and allergens. And possibly new insects. So you kinda need to know what you yourself want out of it. Personally i enjoy the fresh air.