The "legitimate interests" are only legitimate for their and their advertisers data collecting, not your experience (never mind privacy and security). It's just a scummy workaround to avoid regulation.
Even so called "necessary" cookies are often bloated with shit that isn't really necessary (and often leave out necessary functions from where they belong and include them in a separate "functionality" category, where they can pile on extra trackers, especially social media ones).
The worst part is, that some "decline all" don't decline these so called legitimate trackers, so I double check to make sure they are all deselected (sometimes it's an "object to legitimate interests" other times you have to click through to the list and deselect them manually. If it's one of the latter and it won't let me deselect all at once, I leave the site. There isn't any piece of information worth unticking hundreds of boxes for). (ETA: I've found that one of the worst offenders of this is fandom wiki sites, which is where breezewiki comes in and saves the day! If their search function doesn't find what you're looking for, try looking for the fandom you're after + breezewiki on your search engine of choice and if it exists, it will come up)
Never trust companies to have anything but their own interests in mind.