Why is Mrs. Salmon described as the ideal witness by the narrator?

Mrs. Salmon is the chief prosecution witness in the murder trial. At the time of the murder—two o'clock in the morning—Mrs. Salmon was unable to sleep. Suddenly, she heard a door shut and went to the window to investigate. When she looked outside, she saw Adams, the accused, standing on the steps of the victim's house. Adams had just come out of the house and was wearing gloves. Before he walked away, he looked up at Mrs. Salmon's window and saw her clearly.
Mrs. Salmon is regarded as an ideal witness on account of her demeanor. Her slight Scottish accent and her expression of kindness and honesty indicate that this is someone who can be trusted. She has no airs and graces about her, no sense of self-importance. What's more, there appears to be no malice in her.
Juries are supposed to reach their decisions on the basis of the evidence presented to them, especially in a capital murder case. But they are often swayed by the physical appearance and demeanor of those called to testify, irrespective of the evidence they give. Mrs. Salmon is the ideal witness because there's absolutely nothing about her to suggest that she's being anything less than truthful and honest in giving her account of what happened on the day of the murder.