I questioned it also but I guess tabs have become the replacement for bookmarks. AFAIk both chrome and Firefox have a search function specifically for open tabs so for some it's easier to track and store sites that way, especially since the browser can handle it well enough.

It's not my preference as I like having a clean browser window; too many tabs is distracting and it's harder for me to contextualize what work I'm focusing on (windows + tabs for tasks is my preference), but I can see how it works for some.