Maybe they are more important to the club because their money goes directly to the club. Also most are just „lucky“ to have been born / raised in Manchester (unless they moved to Manchester just for United).
Then take me for example, there have been seasons when I watched United at OT 2-3 times per season (on other occasions I can’t manage to go there for two years running: can’t take that many days off work „just“ for United, sometimes I don’t even get tickets for a match even if I could fly on those dates, etc.).
When I attend a game: I get an expensive match ticket, then only once my ticket is confirmed through a ballot or something which I find out only a couple of weeks in advance I can book an expensive flight at short notice. I then pay the hotel night(s). If it is a CL game I also take 1-2 days off work.
So at the end I am paying a similar amount just to watch United only for a couple of times, a season ticket holder pays for the entire season. Obviously the difference is that my money (and time) goes to United, Hotels, airlines, etc. whereas a season ticket holder only invests in the club. And yes he can be there every week obviously.
So I can understand some fans claiming they are more important, but this so called importance is then also based at least partly on luck and / or how „rich“ someone is.