You discover an interesting article while doing something else. Reading it immediately interrupts your work; leaving the tab open clutters your browser. That is why I wrote about Instapaper years ago. A button let you save an article to a list you could revisit quietly.
Finding and reading are different tasks
The main idea is not the button, but separating discovery from focused reading. When you find something, briefly decide whether it is worth keeping. Save it, then return to your task. Make time to read later. A chance discovery can wait without demanding your attention at once.
It helps to clear the collection regularly. Some articles were interesting only in the moment. Delete them. For pieces you want to remember, add a note or save a quotation with context. A long list of unread links is not a knowledge archive.
What Instapaper offers now
According to Instapaper's current documentation, you can save web content, read across devices and archive finished articles. The best browser or phone option depends on your workflow. My old instruction to drag a button to the bookmarks bar is no longer an essential step.
The value lies in a small habit: save selectively, read attentively and clear out the rest.