I usually don't have multiple HN articles open at a time, but I can see how that would just be replacing one problem (too many browser tabs) for a worse problem (too many, now limited, browser tabs).

It's just nice to have HN as it's own app instead of just another tab in a single app. Same reason I use mail.app vs. webmail, native music app vs the web player, etc.

PWAs also solve the problem, more or less, but it is nice to have something native.