Why even bother with a tier list? It's pretty obvious who is dominant and who is not. The lines get blurred the further down you go from there. Just play whoever you do the best with if you want to win, or whoever you want to play otherwise. Is there really a point in having a tier list? I don't think so.
The only tier lists that matter in other fighting games are the ones that top level players come up with anyways, and even then they usually only talk about there top 8 or so, it's not even really a list.
Oh my god, why do people bring this **** up every time we talk about making a tier list. It is obvious at this point that the community wants a tier list. Smashers have been coming up with official tier lists at this point since like 2002, possibly earlier, and I think you've probably noticed that Smashers are a pretty stubborn breed. There will be a tier list, that's just a given in this community. It's just a thing that happens.
In fact, I'm going to widen the bar now and tell you this is just a human thing. I feel like numbers and rankings solidify everything for people. That's why there are so many statistics flying around for everything. Hell, in sports, anything that can ever happen and hope to be objectively recorded has a statistic. There are always rankings for which player is the best in the sport. There are rankings for top everything. That's just how the world works right now.
Aside from that, you tell people to just play with whichever character with which you are the best. But if you are referring to the phenomena in which players pick better characters just because they are more dominant, that will happen regardless of the existence of a tier list. People see that a certain character is dominating everything, it's the same as seeing that said character is top tier. Did you notice the rise of the Diddy Kong army before Smash 4 tier lists by top players became a thing? No tier list, yet everyone is still switching to the dominant character. In fact, making a tier list might have the opposite effect. Especially early in the metagame, people can challenge themselves to develop a metagame, or do well with a crappy character.
And finally, it is a good analytical tool. People who research for the purposes of the tier list are often commentators or content producers in the community, and tier lists are a good way to focus on how certain characters fare against each other, which means that we will get possibly more knowledgable commentary or content. On top of that, it helps newer players to learn what are the dominant forces in the metagame, and allows them to focus on learning those matchups early.