Because we know seeing is believing, and also a much easier way to understand how big they actually are.

5-inch coaster
use to keep drinks off any surfaces you wish

10-inch cocktail napkins
cocktails, appetizers, lunch box, small bites, desserts
can also easily be used as a coaster for drinks during cocktail hour

18-inch everyday napkin
family dinners, informal gatherings, your packed lunch on the go, or really whatever you need it to be, especially if "formal sizing" just isn't your thing

20-inch dinner napkins
formal dinner parties & entertaining guests, it's a "proper napkin" size