“Why are font events necessary for defining fallbacks?”
Sean McBride, writing for Typekit:
It’s always a good idea to pick a fallback font that’s as close to your chosen web font as possible, but there are often minor differences in optical size and appearance. As a result, you may want to serve slightly different styles depending on whether the web font or the fallback font is used, and that’s where font events come in.
Here, Sean explains how font events help us serve different styles for fallbacks. As for choosing the right fallback fonts, a good place to start is this piece by Josh Brewer.