There are a plethora of different materials to be found in Subnautica: Below Zero, and each has its own unique uses. For the most part, a majority of the things that players come across will be used for crafting at the Fabricator or Habitat Builder, but that isn't the primary function of Root Pustules. If these nasty sounding things aren't useful in any kind of crafting recipe, it begs the question of what players would need them for in the first place. As it happens, this material is extremely useful as an energy source for powering players' bases.
In Subnautica: Below Zero, players will eventually reach a point where returning to the same drop pod every time to fabricate things just doesn't cut it. When this happens, the next logical step is to build a base somewhere else, and eventually players may even have multiple bases up and running simultaneously. The catch is that all of these bases require fuel in order to stay powered up.
In order to find Root Pustules, players are going to need to head to the Arctic Kelp Caves biome, as this is the only place where these spawn naturally. As the name implies, they appear as strange yellow growths on the bottom of Kelp roots in this area. Players don't need any kind of tool to harvest them, so the hardest part of getting these Root Pustules is actually finding them. The Seatruck will come in handy here, as it will allow players to travel much faster and go far deeper than they would be able to otherwise.
Although there are many different options available to players for powering their bases, one of the more convenient ones is the Bioreactor. When used in the Bioreactor, Root Pustules provide an impressive 700 energy, the highest amount offered by any item in the game. It's easy to see why they are so valued then, as having a stockpile of them is a great way to ensure that there's never any kind of energy shortage. Nuclear and solar options tend to be better later on, but there are some areas where these types of energy don't work as well.
This is a slight spoiler, but two Root Pustules are also used in the creation of an Architect Component, an item that players will need later on for the story. Regardless of what players need them for, they should have an easier time hunting down Root Pustules in the future.
Subnautica: Below Zero is available now on PC, PS4, Switch, and Xbox One.