Why should an athlete see a doctor after taking a blow to the head?

To check for any haemorrhage (internal bleeding of the brain) or concussion. Even though everything might look fine from the outside, the impact might have caused some internal damage which could have led to inflammation of the brain as well as internal bleeding (usually when the brain bumps against the skull). If left untreated, it can lead to coma as well as other symptoms such as headache, memory and thinking problems, etc.