Modules: Pre-written adventures or settings for use by GMs and players as a time-saving device when setting up and running adventures. Typically, they have a main quest or some adventure seeds as well. They can be very helpful for idea-mining, especially for new GMs, and I have used several in the past.

However, there are some downsides to using modules, mostly as it relates to Game Flow or Feel.

Let’s start with the good aspects:

Pros

• For GMs who worry about making up and maintaining a whole world, it can be very helpful to have shops, NPCs, locations (especially locations which have interesting or unique aspects to them), magic item lists, and enemies already listed and detailed out. This is the big one, as many starting GMs feel quite a bit of pressure to have everything planned. It gets easier the more you run games, but early on it can be very helpful to have these details laid out somewhere, and I was no exception to this rule when I was first starting out.
• Another author’s story can feel fresh, interesting, and inspiring. Spending a lot of time with one’s own story plans can cause a GM to lose confidence in their own writing (“it’s too obvious!” “I don’t think the players will get it,” and “I don’t think my idea is unique enough,” are common feelings). It’s important to keep in mind two things with this though: 1) The players rarely notice, and if they do they will still feel a strong attachment to their own story, even if it does blatantly follow Toy Story 2’s plot or whichever, and 2) You can always change story elements later on, and no one will be any the wiser. It’s a real-time story writing experience, after all.

Cons

• Once you shake the awkwardness of wanting to have everything planned out, modules will hold you back as far as organization. Around the time that it becomes faster to make up your own NPC rather than search the book for the specific shop-keep you need, that is around the time that one feels held back. The human mind, when it has a good idea of what it’s doing, is faster at indexing than the body is at physically searching a book. This could be a problem with some modules in particular which organize themselves in unhelpful ways, but even the best-organized modules will add a few flow-breaking moments to the game if you have to search for a specific detail.
• Modules cannot account for play action perfectly. Some are better at predicting actions than others, but let’s be honest: it’s a hobby where the boundaries are defined only by imagination. There are a thousand different ways players will subvert or change the direction of the story, and unless you or the module maintain a tight hold (running the risk of railroading), you will inevitably come to a point where they go “off book.” Even if it in a small way, like trying to interact with an NPC who is trying to leave the scene, the GM has to improvise at least something to make the game flow right. The more you want to make a module and its world feel alive, the more often players will end up engaging with things not covered in the book, and this can leave a GM feeling that the book is holding back their creativity. Often this feeling comes from trying to stay “true” to the module’s vision of its plot or world, which allows for very little deviation.

Personally, I fly by the seat of my pants a lot these days. A basic outline works for me, and I prefer to let cooperative storytelling take it from there. However I would be a hypocrite if I pretended I didn’t heavily plan my early campaigns and adventures down to the smallest detail. I’ve also used modules, both as a player and GM, and I simply find that I don’t prefer them. But that’s just me! Maybe the upsides for some GMs and players out-weigh the cons at other tables.