why do police turn on there sirens in pursuit of a criminal?

I guess it is because it is in the laws, and upholding the law is important for them to practice. I guess it is also to alert other motorists or people on the sidewalk that they are in emergency, to inform the criminals that they are after. All in all it is included in their Standard Operating Procedure..

Yeah I've been wondering this too, I mean if the criminal hears the siren hes gonna know the police are after him/her and try to get away at all costs. But as the other 2 have already said, they use their sirens to get people out of the way to reduce accidents and other major collisions.

The police (and the ambulances for that matter) turn on the sirens when they are hurrying somewhere (such as the police when pursuing a criminal) so that the other drivers on their lane would know about it and clear the way. :)