How does the inclusion and exclusion of certain evidence result in a fair trial?facts.

This is a pretty broad question, but I will try to give an
answer that can illuminate one part of it at least.

I would
say that a fair trial comes out of the prohibition on evidence that cannot be personally
validated by a witness.  As an example of this, think about the general prohibition on
hearsay evidence.  If I am accused of robbery, the prosecutor cannot call someone to
testify that they heard that I committed a robbery.  The prosecutor would have to call
the person who actually said I committed the robbery.  This gives me (through my lawyer,
most likely) the chance to challenge the witness.

This
requirement that all evidence be subject to interrogation in the court helps ensure a
fair trial.