All buildings have to “breathe” to an extent. It’s possible for a building to be built too “tight.” But, with most homes — and especially with most older homes — it’s the opposite problem that occurs.
Optimally, a house should change out all its old air for new air from the outside about once every three hours. In practice, many homes leak air 2 or 3 times as fast as they should, so all air is exchanged every hour or even every half hour. That makes a huge difference to your ability to affordably control the temperature in your home, or even to control it much at all!
Since hot air rises, the worst air leakage tends to occur in the attic and out of upper-story walls. Some air, however, will leak out anywhere and everywhere if it finds a pathway since air expands and moves horizontally as well as rises.
During windy weather, air leakage will accelerate considerably, making it even more difficult and costly to cool/heat your home and less comfortable to live in.
In addition, air leaks can create moisture problems and indoor air quality problems that will affect your health.