Police machaye shor!

I am not sure if I know the answer to this but it has been bothering me, so here it goes!

Why do police cars have loud sirens when they move to the scene of a robbery? Or do they?

When the siren becomes audible to the robber in the house, won't he start fleeing? Does it not cancel the benefit of rushing faster (agreed, a loud siren helps clear traffic faster)? Am still looking up on it...