Tabs and windows are a very convenient UI for maintaining state. Browsers aren't used just to complete one "task" at a time. The "job to be done" isn't "do online banking" or "buy a bike", it's "organize active information". Across virtual dssktops you might have a "finance" window during tax season with several different sites, a window for each ongoing project with dozens of references queued for each, a window for comms (email, chat, social, etc) with half a dozen tabs, a window for news with all the day's threads in tabs, a window of "TODO" tabs with things to buy, a window for gaming, etc.