In this guide I'll explain how you can achieve this without external apps or pages:
https://psteamcommunity.yuanyoumao.com/sharedfiles/filedetails/?id=3333534392

1. Go to the game you want to farm hours in your library and click on the gear button > Properties...

2. Under the LAUNCH OPTIONS section, paste the following command:

"C:\Windows\system32\charmap.exe" %command%

With this we are changing the app that Steam will launch when you press the PLAY button to the Character Map.[en.wikipedia.org]

3. Finally, run your game. The Character Map should open. Now you can farm hours without wasting resources from your PC.

Tip: If you are farming a lot, and I mean, A LOT of games, you can right click the Character Map icon in the taskbar to close all at once.

• Since this method no longer works with apps you can't install, this is the best way I figured out to track hours.
  
• I'm not really sure if there is a limit of how many games you can play at the same time.
  
• Remember to switch to invisible mode if you are going to farm a lot of games to not annoy others in your friend list.

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• Some people like to compete to see who can track more hours in Steam. For some reason, pages like IdleSteam or FreeHourBoost exists.

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• To easily farm trading cards.
  
• Maybe impress your friends?
  
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• Some hidden apps come without an executable file, just like hackerone_test_app3, and this is the easiest way to get one.