> Sure I get that this is possible given that you know this is what the code does but the bigger questions are who knows that you can do this, and who actually uses this feature?
I do.
> In order get a benefit from this feature you have to be logged into hacker news on multiple devices, collapse a thread on one device, come back to that thread later on the second device and then continue reading from where you left off.
I actually do exactly this. I read on my phone/laptop and later come back to the same thread (when more people have commented) on my home computer, usually at the end of the day. Having threads of conversation that you are not interested in already collapsed on multiple devices is very useful.
> Alternatively you can just scroll past the stuff you've already read, which has a minimal cost to the user.
Scrolling and quickly scanning over multiple threads is more time consuming and becomes actually annoying if you just want to passively follow a thread. If you want to follow multiple threads under the same topic, you either have to remember unique words/usernames/dates of that thread or resort to scroll & scan method mentioned above. One is annoying, the other is simply non viable for N amount of threads.
Until there is a way to subscribe to a thread, this feature is super helpful. Its actually just as good or better than a subscribe mechanism because atleast you can avoid getting multiple notifications for every single comment that get posted in the interesting thread. In the current scheme, all the irrelevant ones are already out of your way.