I don't think there needs to be said anything about Extremes/ Savage, because that's obviously the type of content where a more critical look at your own and your team mates' performance becomes mandatory and ACT (plus fflogs/ xivanalysis) is the only way to really to that. People can feel like they're doing good all day, the analysis often tells a completely different story.
Sure, it's not pleasent to have various tools tell you "you sucked" after you thought you did well, but at least I know what I need to work on and where my mistakes are. Because I don't remember every little thing I did over a 10min+ fight.

If I have a parser running for non-endgame content, it's usually either to get a better overview over how the others play and know who gets my cards, dance partner and priority when raising and adjust my gameplay accordingly or simply to see how I'm doing on a newly leveled job in casual practice. It also helps me handing out comms to someone who definitely deserves it. Bonus points for people who use things like Shield Samba or Second Wind regulary to support the team.
And after running the three experts for who knows how many times, it's fun for me to try and expeririment here and there to see if I can squeeze out a bit more dps by adjusting some things. Optimization in content where nobody cares about is just for the sake of being less bored after running something for the umpteenth time, I guess.