Why is Mrs. Salmon described as the ideal witness by the narrator in "The Case for the Defense"?

In Graham Greene's "The Case for the Defense," there is a man on trial for murder. Three witnesses saw him, but the principal witness is Mrs. Salmon. She is the key witness for the prosecution because she had the most precise view of the murder's face, and she was the one who identified the man to the police.
The narrator describes Mrs. Salmon as the ideal witness:

She was the ideal witness, with her slight Scotch accent and her expression of honesty, care and kindness.

Mrs. Salmon is someone respectable. She is unlikely to lie, and she is genuinely concerned about the outcome of the case. The jurors would automatically trust her, and that is something you want in a witness. However, Mrs. Salmon has a hard time because, despite knowing who she saw, she is unable to differentiate between the defendant and his twin brother. Despite the jury's trust in her, she cannot do her job, and the defendant is acquitted.