If you ask anyone why they use a webcam cover, chances are its for security reasons.
However, Apple is now saying you should remove these covers or suffer the consequences.
Continuing to use a cover could result in a damaged display.

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No green light means no active camera and your activities are safe from prying eyes.
Another tip is to control which apps have camera access, which can be done through the System Preferences.
We design Apple products to protect your privacy and give you control over your information, says Apple.
If it is thicker than 0.1mm, remove the cover before closing your MacBook.
Via:The Verge