The purpose of a court is to determine the truth.
And since we routinely use facial expressions as clues to determining whether a person is telling the truth, or what they're feeling as they speak, the face of a witness has to be uncovered when testifying.
It's as simple as that.
Except that using facial expressions as a lie detector have been as discounted at using a diviner's rod to find water: it is superstitious nonsense.
So yeah, it is as simple as that: seeing a person's face serves no purpose.