Increase public confidence in the system.
If someone is jailed for 15 years they should serve 15 years to the day. If someone is fined £300 they should pay £300. If someone is given 100 hours of community service that's what they should do. No more no less. If the judge / magistrates want to take into account the potential of them behaving in prison just give them less.
Also if someone is say given a jail sentence of two years and a ban on driving for three the driving ban should start after their release from prison.