When setting type on your website, it’s good to know where you can get free fonts. Due to legal purposes many font companies prohibit the use of their font on your personal website. Therefor, when choosing a font to download from websites such as Google Fonts or Typekit, it may not be standard typefaces you’d normally find in your font-book that you can choose from, but there will always be a font so similar that the subtle differences cannot be noticed from a computer/mobile screen.
It is also nice to know what font-family (for example) to use as a headline or what font-family to use as a body. As a designer with previous experience in typography, I didn’t even know that sans serifs were preferred headline fonts. A sans serif is clean and therefor stands out because of how little it needs to fight for your attention. On the other hand, serif fonts, with their little feet, makes it easier for our eyes to follow, which makes it a preferred body font.