ads have two functions: to drive more traffic, and to preserve mindshare for existing brands. Coke is not buying ads because you don't know what Coke is, Coke is buying ads so that Pepsi or somebody else's ad doesn't take your mindshare away.
(Maybe I'm being too literal here -- it'd be fair to argue that Netflix mostly just advertises their shows, not the plans themselves directly.)
[1]: https://www.statista.com/statistics/688525/netflix-ad-expens...