Frankly making fun of Quora and Scribd is like shooting fish in a barrel, but sometimes it's helpful to articulate the obvious.
You may not want to sign up yourself, or you may disagree with this tradeoff on principle, which is fine, but I want people to understand the motivation. We just want to increase the total amount of knowledge shared. In the short term requiring signups means some people don’t join. But a lot of people do join and start to use Quora regularly, a lot more than if there was no requirement, and those people end up contributing so many more answers that we judge this to be the right decision for the long term.
In fact, at this point, most of the answers written every day would not exist if we had not had the signup requirement. I realize that this choice makes the product worse for people who refuse to join. But in the counterfactual world Quora is a product with much less coverage of any topic, and that is a much lower quality experience for users. We are trading off a little annoyance at first for a much better experience and the ability to access a vastly larger base of content in the long term.
Anyway, the point you were answering to specifically mentioned "make it hard to read text on a web page". "music, ebook, software" aren't web pages, it's not the same end goal (the browser displaying stuff on your screen).