I could blather about how Matchup Charts are rather useful in helping you build a team (you're playing Para/BB and just have to look at a few numbers to figure out which char would be a good 3rd to deal with your Fukua problem), or that they make you question yourself when they disagree with what you believe in (eg you think Val loses horribly to Peacock, you quickly check a chart, see that Val wins, now you can go ahead and figure out why everyone else believes that), or that they help you figure out the order of your team;
Or how Tierlists are useful to help you create competitive teams, because sure Anakaris CAN win VSav tournaments, but you'd be insane to believe it isn't significantly easier to do with Sasquatch. Especially for those "High Intermediate" players who would like to push into the "Expert" group, it's highly useful information to see whether their shortcomings are potentially just built around playing a bunch of shit characters that to push farther would require an unneccessarily high workload.
Or how in general, a centralized info source is a neat thing to quickly check anything or lead people to. Or 100 other reasons.
But really, the most straightforward answer to your question -what use MatchupCharts/Tierlists have- is: They create discussion.
Highlevel players get together and discuss what options which character has in what scenario against some other character, and that for every character - against every character.
On a forum, with all of the information getting preserved. And almost everyone who has a vague idea of the game can join in, because even if you're only Intermediate, you will likely be able to throw out some idea on at least one matchup of your main character.
They are an invaluable tool for keeping a community alive (as matchups are always in flux depending on new discoveries, and people tend to disagree on a few things) and in general teach the entire forum various things about parts of the game they usually don't come much into contact with, deepening everyone's understanding and giving them broader horizons. While reading a Matchup/Tierlist thread, you will casually glance over a few discussion posts regarding the Filia vs MsFortune matchup. You would never enter either of those characters subforums, since you don't play either of them. But here, you read the things; and maybe you even remember something of it, and can then either help some newer player with the matchup, or maybe it will be helpful to yourself in the future when you consider picking up a new character.
E: This of course implies that actual discussion (with arguments, and stuff) happens, and it's not just an endless rinse-repeat of
- X poops out a random tierlist
- 5 people say they roughly agree
- 3 people say it's shit and wrong
- Z posts an edit of X's tierlist, where X's main is toptier (up from bad) and Z's main is bad (down from toptier)
- 3 people say they roughly agree
- 5 people say it's shit and wrong
- A poops out a random tierlist