I've seen a few PC box-ish size air purification systems in some acquaintances homes. What is the point in having them, what do they do, and doesn't opening the window for fresh air make them redundant?

Just wondering, looking to improve the air I breathe in Shanghai if possible. Thanks.

PC box-ish size air purification systems

They are probably Blueair air purifiers which just look that way. The air purifiers will mainly remove particles in the air and harmful chemical gases. If your residential house is new or with new furnitures, the air purifiers will be quite useful.