The list of reasons is quite varied. Im only au fait with Boeing. All the Boeing’s I’ve flown have 2 acceptable land flap settings in normal situations. 30/40 in the 737 family 25/30 in others.
a reduced flap setting uses slightly less fuel. Marginal on one approach but when compounded with several a day over a large fleet, it begins to count.
Also less wear on on the flap drive systems (screws have to turn less). But I’ve never found that in any official manual only old wives tales from engineers.
also the full flap setting has protections to prevent excessive forces if the speed is too high. So if you’re heavy and blustery you may be close to the flap limit speed for the full flap setting. If turbulent conditions force the indicated airspeed to go above the allowable speed the flap “blows back” to a lower setting to protect the wing. That will instantly make you “unstable” (a technical term who’s conditions im not
going to bore you with) which will necessitate a go around if youre too low on the approach.
another old wives tale (I say the term in jest because I have no way in personally telling which setting is which from the ground) is that a reduced flap setting is quieter. Less turbulence and less engine noise as the engines will be at a lower setting.
im sure others will add to the list but they’re the first I can think of off hand.
which setting to use generally comes down to SOP, conditions on the day, experience and personal preference of the crew.