Macintosh 1.8 Gamma
The Macintosh 1.8 gamma dates back to the very early days of Macintosh computers, long before color displays and ICC color management was universally adopted. Back then, people found that the best way to get an image viewed on a Macintosh screen to match the output of an Apple black-and-white laser printer was to adjust the gamma of the monitor to 1.8.
These days, Adobe programs like Photoshop and Lightroom always compensate for whatever monitor gamma is used by the system to ensure that all images are displayed at the correct brightness, regardless of the gamma that was selected when calibrating the display. Setting the gamma to 1.8 instead of 2.2 lightens the interface but has absolutely no impact on the lightness of the images displayed in Lightroom. These will always be perceived as being displayed at the same brightness regardless of the monitor gamma.
If you are mainly using your computer for image editing work, it is best to use a gamma setting of
2.2, because it means that the image tones you preview in Lightroom are rendered on the screen with a more even distribution.