pressing the light button once turns on the PARKING lights, at least in ATS. Parking lights are only for when you are parked on the side of the road at night. in real life, it uses too much energy for the low beams to be on (when the engine is off because you're parked), but its still important to be seen, so alas, the parking lights.
General rule of thumb is if you're thinking about whether its dark enough to use lights, then it definitely is.
in the game, depending on graphics settings, the high beams don't blind the other drivers like they do in real life. High beams during night are much worse due to the light difference between the ambient world and the lights, so having highbeams on in the day wont blind people as the outer environment in the day is much brighter than highbeams.
the "light horn" you're talking about is called "flashing your high beams" its totally legal and acceptable, and its usually only used to tell the person in front that they can merge in front of you (if their lane is ending or if they have their signals on to indicate they want to change lanes). You should not use it as a horn though, use the actual horn for that.