Your operating system uses a font which the design team behind your poison of choice believes gives you the best possible font designed to be the most legible and easy to consume content for that particular OS (and sometimes device as well).
This script forces the usage of these fonts universally.
NOTE: Fira Sans i.e. the fire OS default font is part of the 'Roboto' family. Furthermore, the iOS 13 default new york font is not yet available as a web font.