Modding and testing, you pretty much have to.
For gameplay, I don't tend to on first time or on replays if it's a good game with nice replayability.
Though lately, I have done it either because I don't want to spend all that much time on a game and it makes no difference to the result OR it's a game that grades your ending based on performance and it's nigh impossible to get anything better than "D" ranking or similar.
I have done it before and taken shortcuts and the like just because I'm somewhat interested but it isn't worth taking the time and going to all that bother. Or if it's the case where you cannot get hold of some items or characters through playing but it's there in the data. Or attaining such a rare item it costs you an inordinate amount of time IRL playing just to maybe get the opportunity to get it which might not even pan out at all.
Or cutting down inconvenience. I.E. Megaman: The Wily Wars. Say you already played & beat MM1, MM2, & MM3 on the NES, You're not interested in doing it again for Sega Mega Drive. You just want to go to the Dr. Wily tower game.
We'll murder them all, amid laughter and merriment...except for the few we take home to experiment!
"I cant see S***! --YOU GO TO HELL!" --Tourettes guy