I'm not sure why you're confused. None of the advice here fully contradicts each other.
Because every instance is different, and every party is different. If people are taking a while to kill trash mobs, and it's an instance where there's a lot, of course it makes sense to use it.
But if the trash mobs die very quickly without it, that'd be a waste. So for most content, it would be a waste.
Using it for the first boss was already stated that it's because for some instances there's enough time to build the gauge back up. So if you can get 2 limit breaks, it makes sense.
But for a lot of instances, that won't happen - so saving it for the hardest boss (which is -usually- the last one) makes sense.
Saving it for the healer, great for people progging savage content (or pugging, because you never know what you're getting with pugs). It can save a run. For more casual content? Probably not, because you shouldn't need it. But obviously if you're having trouble staying alive, there's no reason to save it for a damage limit break because you might not live long enough to make use of it.
These aren't contradictions, they're conditions - it's called "strategy". Being confused by this would be like being confused about using fire 4 vs fire 2 - one being best for single target (4), the other for lots of monsters (2).
If you're expecting a "do this all the time forever and ever" answer, of course you won't get that -but I don't get why you'd expect it to be that simple. In that case SE wouldn't have even bothered making different limit breaks if only 1 would ever be acceptable for any and all content.