Bleeding the brakes is an inevitable part of DIY automotive repairs. Air can enter the brake system during repairs, but the more common reason is far more insidious. Brake fluid is hygroscopic and readily absorbs water from the atmosphere. Water in the brake fluid lowers its boiling point. Heat boils the brake fluid, and the resulting steam leaves air in the lines. Brake fluid is incompressible while air is easily compressed and manifests as a spongy brake pedal or worse.
This is a companion discussion topic for the original entry at https://www.hagerty.com/articles-videos/articles/2019/06/12/5-ways-to-bleed-your-brakes