Why is discretion a necessary feature of the criminal justice system?

Discretion as in Prosecutorial discretion is a must. The
law should not be able to force a Prosecutor to prosecute ALL
cases.

Judicial discretion is based on a possible sentence
based on the defendant's prior conduct and recidivism rate,
etc.

Police discretion is, unless the law mandates an
arrest, it is generally discretionary for them to arrest on the spot. Also they have the
legal discretion to issue a citation/ticket or not, except when based on race. color,
etc.

These, I see no problem with.