Good pairings help make for a complete, thoughtful experience. The icing on the cake, if you will. Though cakes, with the exception of coffee cake, need icing. They just do.
Now that you’re thinking about cake, let’s jump to dinner. Dinner is fun! Especially when you’re able to consider a wine to drink with your dinner (perhaps pairing acidity to heaviness), or a beer to drink with another dinner (bitter and hoppy plays well with heavier dishes). Pairing can elevate a meal. Similarly, well paired fonts can make a good design.
The purpose of pairing fonts is to improve readability and establish hierarchy. The classic serif-with-a-sans-serif combination is used on print and web, often with the headline as the sans-serif font. It works because it grabs attention, but more importantly, it’s easy to read.
The ease-of-reading factor is key in font pairing. (And ideally, in choosing a font in general.) When readability becomes an issue, communication becomes an issue. This is to be avoided. Using fonts that are legible on screen or when printed is always a good place to start. If you find a font combination that works both online and in print, you have made magic and deserve cake.
But note! Two fonts looking good together does not equal good design. There are images and layouts and format to consider. Consult your local graphic designer for more information.