Really?

I have seen this sign on the back of a few vans - "Police follow this van". Each time it has set me wondering why?
- can you just ask the police to follow you around?
- Do the police follow the vans and if so when do they stop?
- if all the vans stolen or not have this sign whats the point?
As a result I have placed a large sign on our house "police make sure we don't get robbed" I think it will do the job.