The players only have to worry about one (or maybe a few) characters. Meanwhile, the GM has to worry about literally every other person in all of existence in the world they're playing in (plus all the planes). That can be quite a task. Thankfully, there's things like the Castles & Crusades NPC Almanac here to help them out. The book is up on Kickstarter now.
From the campaign:
The NPC Almanac is your complete guide to Non Player Characters. Designed for use with the Castles & Crusades RPG, you'll find that it works with just about any role playing system. In the Almanac you'll find three different approaches to non player characters: the shot, standard and long (see below for more), this approach brings to life hundreds upon hundreds of NPCs for your favorite tabletop game. The Almanac is roughly 175+ pages long, will be produced as soft cover, black and white book. It is text heavy. The print copy will retail for around $25.00, the PDF for $15.00 or thereabouts.
The campaign's roughly 5x funded with 24 days left to go.