We highly recommend that you dry your resin before use, it will allow you to achieve much higher filament tolerances.
Most plastics pull moisture out of the air. They are “hygroscopic.” Even if your material supplier dries the pellets and immediately packs them in plastic bags, the resin will still contain moisture when you get them. Polyethylene-based bags do not block the moisture in the air from getting to the pellets. Polyethylene by itself is not a true “barrier” resin; it is “porous” to moisture and humidity.
Once you dry a polymer you generally have to process it within 30 minutes. Once exposed in the hopper to ambient air, it will again magnetically attract the humidity, needing then to be re-dried before processing.
Some moderately hygroscopic resins like ABS and acrylic can be processed wet, but the moisture will turn to steam and cause splay. The moisture does not break up the backbone of these polymers. With other resins—like PET, PBT, PC, nylon, and TPU—exposure to moisture at processing temperature causes a chemical reaction (hydrolysis), which breaks long chains into shorter fragments, reducing strength and other properties. The “broken” polymer also flows easier, so if the polymer is not uniformly dried, your process must contend with variation in viscosity of the incoming resin at the feed throat.