[Derek]

There are a couple of reasons for this.

Firstly all officers trained to advanced driving level (blue lights etc.) now have to be reassessed on a semi frequent level so there are more training runs being done.

Secondly a significant chunk of other road users are completely useless and don't see or hear a police car coming up behind them. Rapid changed from long tone - short tone - off - short tone get the best reaction from drivers.

Also sirens are LOUD. Trying to use personal or car radios while something is screaming just outside the car can be a pain in the rear.

Finally it's a matter of judgement. Going to some calls you want a silent approach so not to scare people off and at nights going through residential areas having sirens on will almost always result in complaints, it's far easier to put them on for junctions only.