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The reason you dry certain plastic shredder  is to get the moisture out. But why does the moisture have to be taken out before processing? Even though some chemistry is involved, we are going to explain this with the analogy of tiny magnets. Everybody understands magnets.
 
First: Plastics get their strength from long polymer chains that are entangled. Short chains provide easier flow but sacrifice strength and other properties.

Second: Some Plastics pull moisture out of the air. They are “hygroscopic.” Even if your material supplier dries the pellets and immediately packs them in plastic recycling machine, the resin will still contain moisture when you get them. Why? Because 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 vapor (humidity).

Third: Why are some resins hygroscopic in the first place? Polycarbonate, nylon, PBT, PET, ABS, acrylic, urethanes, and many other hygroscopic resins are long-chain polymers, as are polyethylene, polystyrene, etc. Hygroscopic resins contain carbon and hydrogen but, also have a “polar” segment of atoms that behave like a small magnet with north and south poles—i.e., positive and negative charges. If moisture, also a polar substance, is present, you get two mini-magnets that attract each other.

Fourth: 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 plastic crusher 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. There may be no splay visible at low levels of moisture in the resin. The part will look fine and may even pass tests at room temperature, but will fail prematurely at low or high temperatures.