As demarcated in this image. What is its utility?
Probably not a complete answer. It searches the same title and displays the results.
I went to this page and clicked on past. It searched the same title i.e The Nobel Prize in Chemistry 2020 and there were past results as well. See screenshots below :
According to the about page on Algolia, where it redirects after clicking past
HN Search provides real-time full-text search for the HackerNews community site. Source code is available on GitHub. The search backend is implemented using Algolia instant search engine.