A lot of wordpress themes are coded to use specific fonts that the designer likes. The good news is that it is very easy to add this feature to your wordpress sites.
Benefits of using Google Fonts
Your site will show look the same on any browser as most of the Google fonts are now universally used.
I was looking for a font to use in a new site and I wanted Gil Sans which is one of the original sans serif fonts. It is not easy to find so I looked for another san serif font that is close to the one I wanted.
You can easily change the fonts used by using a plugin like WP Google Fonts to choose your own font. You can choose to download a copy and upload it via FTP or just add it from your WordPress dashboard.