Meta Viewport Adder от Heinrich Lunge

Meta Viewport Adder adds the <meta name="viewport"> tag with the basic values width=device-width and initial-scale=1.0 of the content attribute. This allows sites to fit the width of the device and have a standard zoom.

Why is it only added to sites that don't have it?
Because it could generate unwanted changes in the interface of a site that already has a different setting.

If you want to check the source code, you can find it on GitHub.