No, you don't “need” a grinder when using a smoking pipe, but it's highly recommended. Grinding your cannabis provides a consistent texture, ensuring even burning and a smoother smoking experience. If you don't have a grinder, you can break up the cannabis with your fingers, but this might result in larger chunks that don't burn as evenly. Using grinders also helps to more efficiently utilize the material, as it increases the surface area that gets exposed to the flame or heat.