Mainly this is most important to app design but not a bad idea for any type of screen design. You can find many articles that discuss this better than I could. In short, it speeds up your workflow and ensures consistent look on most screen resolutions. In my experience it also has an added bonus of working well with icons that often tend to be in 16 or 24px grids.
This has a good breakdown if the 8-pt grid which most other systems reference: https://spec.fm/specifics/8-pt-grid