So back in the early days of WoW raid bosses were far more simple to now. This was due to Blizzard still being relatively new to mmo boss design and also because of add-ons. In the beginning there were only a handful of add-ons but as time progressed they became commonly used, one of those being boss add-ons. What they do is put timers on the screen showing the queue of boss abilities giving you plenty of time to know exactly what was going to happen and exactly when. Unsurprisingly this made bosses easier to do so in reaction to this Blizzard had to make bosses harder because an enormous chunk of the raiding community were trivialising them with add-ons. Eventually it got to the point that any remotely serious raiding team used boss mods.
And this created a vicious cycle. Players use boss mods to kill bosses faster so then Blizzard creates bosses with the use of those add-ons in mind so that they remain challenging even with add-on use, which means you actually need to have a boss add-on to be able to do the content in a timely manner.
Having a damage meter was mandatory in every team I played with. Not only because they showed dps, but they also showed who died of what and when, what cds were used, who got debuffs, who cleansed them...so much useful information that is a lot more accessible than going through WoW's version of fflogs.
A threat meter is often mandatory so you can ensure that you never pulled aggro off of the main tank by accident. I don't know if this is still the case now but back when I played it was a lot easier for dps to get aggro mid-fight than in FF, so most dps needed to have one especially if they had high burst damage.
Some guilds used player generated currency to divvy out loot. You bid on item drops and you accumulated that currency with raid attendance. As far as I know loot is personal now in raids, (loot is automatically and randomly given to players by the game, you don't roll) I don't know for sure if it is because I never needed to check as I had no intention of raiding when I briefly played again, but if this is the case now then add-ons that managed that are no longer needed.
Some bosses have certain mechanics that have add-ons specifically developed for them. If you're in a competitive team you're expected to use every edge possible to ensure faster kills, so if a boss specific add-on was helpful in clearing it faster you are most times expected to have it.
So the above is what was typically mandatory back when I raided. In addition to this most players needed additional add-ons to make their ui more suitable for the class and role they played. And as I mentioned before in WoW's base ui you can't even move your action bars. It was pretty rare to find someone who raided regularly who didn't use add-ons to make their ui make sense for what they had to do.
Yeap sometimes the dev for the add-on would update it and break it in the process, which was very frustrating if you had a raid before the add-on got fixed, so you had to find an older version of it to roll-back to before it was broken or try to find another similar add-on that worked.
The worst was when you found a great add-on and then after some time the dev, for whatever reason, stopped working on it. If no one picked up that project then you would eventually have to stop using the add-on because as time passed it would become more and more out of date which meant a higher chance for ui errors. And sometimes you can't find another similar add-on that do the things you need from the other add-on...so then you just have to deal with being without something that improved your experience in the game.
Is FFXIV's ui perfect? Absolutely not, but because SE update it regularly and its base ui gives a tremendous amount of options for players to tweak the layout to whatever suits them best, you can still do high-end content without your ui getting in the way of your combat performance.
Personally the only thing that's missing for me is something small about the focus target pane. I wish it showed the target of the focus target...but that's it. Aside from that one small thing I don't ever feel like I need an add-on to be able to play the way I want. FFXIV's ui options fulfill every other need that I have, and that small thing it doesn't has very little effect on me. I can easily deal with not having it, I would just prefer to have it.