I don't know why people always say this, it's factually inaccurate. Apple supports clamshell mode just fine — IE when you plug the computer into an external monitor and close the MacBook lid so that only the screen on the external display is in use — and if there was any validity to the argument that you'd damage the computer shutting the lid they, themselves, would say not to do it.
Prevent Mac from sleeping. As mentioned earlier, OS X’s native feature to prevent App Nap may not be available for all the apps. If you want to make sure a particular app is not paused automatically, but it doesn’t have the Prevent App Nap option in its Info window, there are some third party alternatives available to help you achieve the same. How to Stop Enhanced Notifications Waking a Mac from Sleep Dec 20, 2017 How to Prevent MacBook from sleeping while Downloading a
How to Stop Your Mac Sleeping While Time Machine is Running
Jan 30, 2017 How to Prevent Mac From Sleeping When You Close the Lid
How to Stop Your Mac Sleeping While Time Machine is Running. You can tell Power Manager not to put your Mac to sleep while Time Machine is running. In this recipe, we walk through how to add this condition to your sleep event. Time Machine is macOS's included back-up solution. Time Machine works in the background and quietly maintains a copy of
My iMac falls to sleep when I walk away for a while. Even when the system should be busy with downloading files and converting videos. Either way, you can still let the display sleep to save power and the computer will look like it's sleeping. Specific to AppleTV streaming, there appears to be a bug in iTunes 11. Here's an Open Radar bug