Websites exist to be visited, but visits can mean different things for different sites.
For media sites, for example, traffic can be a source of income from advertisements. For ecommerce retailers, traffic translates to direct sales.
Some sites might not be selling anything, or they might be selling a brand where just having eyeballs on your content is enough to count as a win.
No matter what your site is about, traffic = customers.
And in search engine terms, ranking = traffic = customers.
So when your search ranking drops – or your traffic drops – that’s a problem.