Editor Styles serve a aesthetic purpose, by making the visual editor more pleasing to look at, but they also give the person creating the post or page a better idea of how the content will look when it is published. For example, without editor styles, titles that are displayed on a single line in the visual editor might end up line breaking on the site because the font and font size are different.
By default, the visual editor in WordPress is set in the serif font Georgia and has dark blue link colors. The WP Glossary editor styles replaces Georgia with the macOS system font San Francisco, and changes the link color to purple, in order to match the design of the site.