Prior to the schedule change, the NFL preseason was much more predictable. The third game was usually treated as the “dress rehearsal” game for starters, with nearly every team holding out starters for that fourth exhibition game.
Now, with only three preseason games, schedules have changed. Teams are exercising a ton of caution with their starters, and joint-practices have become a more viable option for getting players reps in a more controlled environment. Rather than rely on an officiating crew that is in preseason mode themselves, coaches can control the whistle during practices, limiting some of the punishment that is dealt during a typical game.