By driving often near my local airport, I deduced that runway lights are kept always on as soon as the day light or weather conditions require it. I also suppose that it comes from safety reasons established in the past ad that's a worldwide standard. what I don't understand is why, given nowaday's instrumentation both on ground and airborne, it's not possible to switch them off when no aircraft is ever going to even fly at cruise altitude around it. Here at LIBP (Labruzzo Airport, Italy) we hardly have more than 8 transits during winter so probably 80% of the energy spent by lights it's totally wasted.
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.
Large commercial aircraft have lots of instruments and navigation equipment but small private planes aren't so different from your car. If a plane has an emergency, you want it to be able to be able to tell where the airport is without the airport having to figure out that something is going on. Obviously, a pilot will tell the airport about an emergency if they can, but what if you have a small plane whose radio has died? They're flying around at a few thousand feet looking just like any other small plane, and there's no real way for the airport to figure out that somebody wants to land, if that somebody can't see the airport.