Why use multiple fonts in font-family

All fonts be a family fonts not a single font because we use "font-family" property in the class. So family means many fonts to be in a family. If one family font not found in your user system's font then it's moved the second font and then it's moved the third font and continue. If not found any defined fonts then show default font as browser using and your design will not show looking good or weird.

Also, we are using a third party font as Google fonts plugins. If you will use third-party fonts then it shows always the same as you have defined fonts.