I can't speak for the other commenters, but for me it's because my browsing style was formed back in the days of dialup in Australia. You'd click a link and wait 30+ seconds to have the page load, so instead of staring a loading page you'd open anything that looked interesting in a new tab and continue on the current page, so all the interesting links would be there ready to read without waiting. invariably you wouldn't end up having enough time to read them all, but they piqued your interest enough to open them, so you'd keep them around in the hope that some time later you would. Rinse, lather, repeat, and add in persistent browser sessions between restarts and suddenly you've got 100+ tabs open.

The internet is much faster these days, but I still open way more links than I actually have time to read because they sound interesting

Use case that I'd guess many on HN would relate to: research and source documents.

When researching some topic, it's common to have the source document or documents open and then other tabs where people comment on them.

For example, if I'm checking some Python docs, I'll sometimes check how more knowledgeable programmers have solved some edge cases.