100 Monster Hunts is a simple idea. As a Dungeon Master, you can roll on a giant table and get a random "monster hunt", or a small designed encounter with a monster. Players then go deal with that monster. It's clean, simple and helpful.
Every DM runs into moments where they need to fill time or generate some kind of hook to get players interested in a local area. It's always a little clunky when your players hit a new town, go to the inn and then hear a dramatic monologue about why the local Baron is definitely a denizen of the Dark Lord who has to be slain so that the Free People might prosper again.
You have to warm players up to a locale, get them invested in that innkeeper and then you drop the Dark Lord bombshell.
100 Monster Hunts is designed to fill in that space so that you don't have to. As it says in the official description for the 5th Edition Dungeons & Dragons supplement, it is meant to help with "small bounties, quests or plot hooks when populating a town, village, city or general questing noticeboard".
They're clean, simple infestations of monstrous creatures for players to hunt down and they have a perfect tone to them that would make a The Witcher 3 quest designer proud.
"The Baron of Butcher's Bay", for example, is a clean vampire hunt that gives you a wounded vampire, a vampire hunter who needs someone to take his place and a context in which those things happen. The vampire knows you're coming, it's going to try to hunt you via its spawn and the players have to figure out how that works.
As a DM, I can think of a dozen ways to connect that to a campaign or help spin that into the game that I'm already playing, but it's also great because it can truly be a side quest distraction that takes up a single session so that my players can have a solid encounter with a beginning, middle and end where the entire game world isn't at stake.
And, even better, you could just use this as a way of bootstrapping a solid Witcher-focused campaign where your players roll around the game world and slay monsters for fun.