Why are runway lights always on?
The answer to the question lies in your statements


  I deduced that runway lights are kept always on as soon as the day
  light or weather conditions require it


Typically when the visibility drops below a certain level airports will turn their lights on. You mention that the airport does not have a whole lot of arrivals but you would only be able to progressively turn them on and off if all the arrivals were planned. There are plenty of cases where they may not be, (here in the US GA traffic is one), emergencies, diversion, aircraft not operating on a flight plan etc. In that case its simple courtesy to leave them on. 

In some cases the lights are actually part of the instrument system and the regulations may dictate that if the ILS is on so are the lights. 

The lights may also not always be on, it may simply be that you happen to drive past them when they commonly are on. There are lots of variables here but different airports handle this differently. Here in the states smaller non-towered airports often have lights that are always off, a pilot can turn them on by keying the mic button on their radio 3 or 5 times. They are typically on a 10 or 15 minute timer. If you turn them on to far out and forget about it they may shut off on short final... Larger towered airports may keep some lights on and allow you to request others be turned on or off. This is only applicable if the tower is in operation. At night, if the tower closes they may simply leave the lights on as there is no way to remote key them.  

At night, even in good weather (or especially in good weather) lights can help identify an airport from quite a distance. Since runways are lit in a predictable, standard way if you are on approach to an airport in a well lit city it can help to split the airport out from all the roads and highways when looking for it in the terminal area.