Vandalism

Last night i returned home to a note from the police telling me my car window had been damaged.
It's cost me £70 and a day of phone calls and missed social events. All for nothing. Nothing was taken. It looks like they went through the car looking for change or something.
Why do people do this?
I have to say the police have been excellent. They replied to calls etc and seemed to take it all very seriously (this is Lewes after all)