Relying on systems fonts greatly improves page performance and can also help your web application/site blend in with the user’s operating system.
Readability is strongly linked to familiarity, so this helps create a more fluid reading experience, allowing your users to focus on your content. Varying the font-weight of different pieces of text can help create contrast between pieces of information.
It can help call attention to a piece of content, or help to make a smaller font-size a bit more readable. In Elements of Typographic Style, Robert Bringhurst suggests we keep two things in mind when playing with font-weight. Use bold weights sparingly. Font-weight should decrease as font-size increases.