Why can't you just open the window?

With traditional ventilation, the air temperature in the apartment is equalized to the outside temperature, which can be especially uncomfortable in the winter cold and summer heat.
The HRV allows you to maintain a microclimate in the room almost unchanged, while the air inside will be fresh. In addition, proper ventilation, which really removes all excess moisture, involves opening all windows wide open at least 3 times a day, which few people do.