How would they be used to help steal a car? If your car is parked somewhere they can stash a tag in it without notice, why wouldn't they just steal it then?
Don’t waste time trying to make sense out of someone else’s nonsense especially when they’re a criminal.
It takes time to steal a car or car parts so maybe they were looking for a better time.
Perhaps they wanted to know where the driver lives to burglarize their home.