Law enforcement might not be the only use for this (although it's the one that springs most readily to mind). E.g. in London there's the congestion charge, which would probably be much more effective in reducing congestion + emissions (or at least loading more costs onto those causing the issue) by charging per mile instead of a flat per day fee.

A system like this would make that possible without requiring a tracker to be fitted to every car, although a growing number of people have these anyway for a reduced insurance premium.