They could be working for a number of authorities - there are a number of independent companies that carry out these surveys, such as this one:
http://www.trafficsurveys.co.uk/services.php#1
They are used to help councils decide if a road needs widening; or traffic lights or a roundabout; or speed restriction; or weight restriction; or speed humps; or strengthening.
Monitoring traffic can also help with air quality issues; parking restriction planning; pedestrian needs such as a bridge or a pelican crossing.
They may be recording the number of foreign licence plates.
Emergency services also need this information so they can plan strategies for getting to an emergency.
Why they annoy you, I don't know.