That's why splash pages (and writeups, like the original) are important. But as mentioned in my previous comment, those splash pages/writeups also have to be effective advertisements.
without any explanation of why I should use your app,
or even how I'm supposed to use your app, I would
close it immediately.
It wasn't until many months later, after experience with other sites with similar interfaces, that I actually started using HN because I then understood how and why to use it. The HN interface is intuitive enough and similar enough to other website to understand that you're being shown comments and a reply box when you click on a link.
That's not the case with this [1]. If you linked me directly to a page that looked like this, without any explanation of what these boxes of text are telling me, where they are coming from, or what they are, I would not use it. The button next to them (a grey box with a plus symbol on it) doesn't explain to me at all what it does, and is not something you see enough in other apps to make a good enough guess as to what it does. After reading the blog post, my assumption is that clicking that button is supposed to send me an email, I think? But I would have absolutely no way of knowing that if you just linked me directly to the app.
1: https://hackernewslater.com/images/hacker_news_later_screen_...
edit: if you download the actual app, the first page it shows you actually is a splash screen that gives a brief explanation of what the app does and why. If that same splash screen content had just been shown and linked to at the top of the original writeup, I suspect he would've gotten a lot more installs.
The button next to them (a grey box with a plus symbol
on it) doesn't explain to me at all what it does
I would expect over 50% of users to take that first step. That is what I saw in similar scenarios.