I agree with you as a user. From the company's perspective though, I'm guessing there's a decent correlation between "people who will pay $, instead of free" and "people who have enough disposable income to impulse-buy high-cost items from your ads".

As anecdata, if you offered me instagram without ads for $10 / year I wouldn't even blink before accepting it. Yet I've also bought a half dozen things from their ads for > $50 over the last few years, which is likely more valuable to them. Or more specifically, makes their ad platform more valuable to their ad-buying business customers.