Starters have to play, and that makes it serious business.
Once upon a time NFL teams played six preseason games. The games were necessary and important for two reasons. The players needed them to play themselves back into condition. Most of them didn’t make enough money to last through the calendar year, so they had to take offseason jobs to make ends meet. They were too busy working to stay in shape.
But the main reason was because the franchises were using preseason games to bring in extra revenue by moving them to non-NFL cities.