Feedback is key to improvement. Objective feedback is rare.
Your mind is subjective
The human mind is an incredible distortion machine. How you think you are playing is often quite different from how you are actually playing.
You might think you hit a fast forehand. You might think you are playing badly or playing great. The challenge is your perception isn’t accurate.
People are biased
You could ask a friend for their opinion. Although they will be more objective than you, every person has their own, often subconscious biases.
The camera doesn’t have feelings
A camera is objective. It doesn’t care if it hurts your feelings.
If you play a sport and want to improve quickly, film yourself.