What is the purpose of ventilators in house when we have windows?

To circulate air and prevent moisture buildup.

New homes have become too efficient at saving energy. They tend to trap everything into the living space to save the cost of heating or cooling it.

This means that moisture gets trapped too. Trapped moisture leads to mildew and mold. Vents in the crawlspace and attic help get some of that moisture out.

Smells, odors, and waste gases also get trapped inside. These must still be vented outward for our safety and comfort.

Properly equipped ventilators remove excessive hot and/or moist air trapped between the insulation in the ceiling and the roof. Moisture breeds mold and mildew (not healthy). Roofs do a great job of collecting heat and transfering it to the attic space and making it much hotter than the ambient air, reducing the lifespan on roofing materials and placing a much warmer mass of air near the living space. Exchanging this air regularly will cool this space and make the living area much more efficient to cool and increase the lifespan of the roofing materials. To give you reference, it can be 90F outside but in full sunlight an attic space can get much over 120F. A ventilator with the right airflow for the square footage of the house can reduce this temp significantly.