Windows 7 introduced several new power-saving measures in order to extend the battery life of your laptop. One very visible feature is the way the laptop screen dims after a set period. These power-saving features have been further improved upon in Windows 11/10/8.
Why does laptop display dim automatically?
Adaptive Display Brightness
Windows extends battery life for your laptop with an Adaptive Display Brightness feature. This feature automatically dims the display on your laptop when it notices that your computer has not been used for a while or after a set period of time.
This is similar to what most cell phones do.
You may also be aware of the process of managing power settings and creating custom plans in your Windows PC.
But if you want to learn more, you can read this post on how to Conserve Battery Power and Extend or Prolong Battery Life.