|An internet webpage is designed to use certain specific fonts: the page's coding tells your browser what fonts the browser must use.|
Thus it is decided by the author of the web page what fonts you see when you view that page. The choice of fonts is not user-selectable!
The choice is limited, in practice, by what fonts you actually have in your computer. The web page usually gives your browser a list of fonts, not just a single one: if you don't have the first choice font, your browser will use the second choice, etc.
If you delete a font that you don't like, that does give you a limited degree of choice: the browser can't use the deleted font, so is forced to use the next font in the list which the web page has specified.
The fonts in your computer are also there for you to use in word processing programs, such as Microsoft Word. In those programs you get to choose which font to use when displaying or printing your documents.