We have only heard , heard is a gross exaggeration in this case, the prosecution arguments.
It is the job of the blood spatter expert to determine how the attack played out, the force and velocity of the blows and the proximity of the assailants to the victim.. and hopefully identify which was the fatal blow and who inflicted it.
He makes the story of the massacre understandable through the use of angles, directions, where the blood fell, its type and the injury which is likely to have been caused by each blow.
It matters little what they testify.. scientific principles will out the truth. Its a pretty exacting science.
He is the third person that knows what happened and the sequence in that room that night.
His testimony is the most important testimony in that court, he is in effect a witness, like a cctv camera witnesses.