I enable them when it is a language I don’t understand (if spoken language in certain scenarios are not by default of the game shown to the player without the need for subtitles to be enabled). Sometimes crucial information is revealed.
I feel that subtitles breaks immersion, also force of habit compels me to read them, even if the spoken language is English, and it’s clearly audible.
I tend to leave them off if they are not on by default - since I read faster than the characters speak and sometimes the subtitles can be “spoilerish” if they don’t take into account pauses or action on screen.
Most RPG’s have subtitles in dialogue and so sometimes I’ll skim read and advance the conversation ahead if it’s not too gripping.
It depends - I have learnt to leave them on, but then sometimes I get frustrated cos I read faster than the cutscene - it drives me insane and then I tend to skip cutscenes - so these day if its a first play thru I leave them off.