You'll be amazed to hear that there are people who make the decision of whether or not to stay put in a website, based on the amount of traffic it gets. Is this a good way of validating a site's content? People actually use various tools to ascertain this and to some extent, I think they have a point.
Looking at the amount of traffic a website gets looks like a good thing but I don't see if this has much significance to someone who's just going to a website for the sake of it. It seems more significant to a visitor looking for the most popular site in a certain niche.
I totally agree with you here, but what about the visitor that would be doing so say, for research purposes and would be looking at a website's significance, based on the number of people who already know about it?
There is that. But if they do that, wouldn't they also be looking for the amount of upvotes or likes or positive reviews that the site received? With this, relevance walks hand in hand with legitimate, truthful and useful information.