Coming up to the truly difficult letters now, and I'll just point out the obvious with this one, since we all know what a witness is.
Witness is defined as: one who testifies or is legally qualified to testify in a case or to give evidence before a judicial tribunal or similar inquiry.
A witness is often somebody who witnessed the event - commonly a crime or an accident. This is specifically known as a percipient witness, or an eyewitness. They can give an oral or written statement.
When giving a witness statement to the police, they will keep your details private to protect you.
It is against the law to intimidate a witness.
The protection of witnesses is crucial in society, without it people who witnessed crimes or accidents would not report what they saw because they are afraid of being targeted. The protection of witnesses means that people will be more likely to help the police with their investigations, which in turn will assist the police to successfully convict people and distribute justice to those who commit wrongful acts. It may have a more positive effect on the community - making people feel safer, reducing crime rate and thus benefiting the area people live in.
From the top of my head, that's all I can think on that topic. :)