Carbon Fiber Reinforced PLA is a stiff material and makes parts that are very solid. It is great for printing high-strength mechanical parts. Increased rigidity from the carbon fiber means increased structural support but decreased flexibility, making it ideal material for frames, supports, propellers, tools... anything not expected (or desired) to bend. It is particularly loved by drone builders and RC hobbyists. The carbon fibers are processed to an optimum size: short enough to print in PLA without clogging nozzles, but long enough to provide the added rigidity carbon fiber is famous for.