Even so, two parties who trust each other can use them without calling the police about it. (Actually a post-dated check is not so much illegal as it is invalid and therefore not intrinsically worth anything.)
I worked for a major company which operated in all 50 US states about ten years ago which paid me via post-dated checks. I had to wait a few days after receiving my check in the mail before I could deposit it.
You can preemptively write cheques for future payments, like buying a car in two payments: one now and one in 3 months.
It also works for guarantee deposits, or write blank cheques with only the receiver filled in, etc.