I think that there's a few reasons why a lot of emphasis is put on sales charts.
One is that it has a certain objective quality to it.
What I mean by that is that it's numerical. It's math. And math is one of the most objective things imaginable. And it's math based on something concrete and of tangible value - The number of copies that fans were willing to part with their money for. That gives it a certain weight that a numerical rating doesn't. Me giving Madoka Magica 9/10 means a helluva lot less than 50,000 or more fans parting with their money to buy the PMMM DVD/Blu-Rays.
So there is value to selling a lot. But I do think it's possible to overstate it.
Here's my general take:
If something is selling a lot, it's doing
something right. It's filling some sort of desire/need in a way that resonates with paying fans. That doesn't necessarily mean it's thoroughly great (and it could even be deeply flawed), but it's legitimately strong
somewhere. I will admit I enjoy trying to find out what "strong kernel" makes shows I'm not that fond of sell well.
If something is selling little, it's doing
something wrong. It either lacks a certain key element that inspires people to buy it, or it has element(s) that are turning people off. Sometimes this might be largely a failure of marketing, but there's some weakness there. I will admit I find it interesting to try to figure out why a show I liked
didn't sell well.
But beyond that, I don't think that sales charts indicate much about quality. I certainly wouldn't think that the five best-selling anime shows of the year are automatically the five best anime shows of the year. There may well be some overlap, but I don't think that sales charts are
that tightly tied to quality.
The greatest value I see in sales charts is how they tend to be predictive of where the anime industry will (eventually) go next. LN and VN adaptations didn't sprout from out of nowhere, as successes like Haruhi/Shana/Monogatari's and the Key titles helped pave the way for them. Certain popular characters (and character types) tend to drive future productions too.
Some characters even get expys of themselves created
...
