[falcor84]

A couple of decades ago, browsing the web was considered a specific "activity" that you do on a computer for a specific need, and then and close the browser window when you're done.

A few decades earlier, using a personal computer at all was considered to be a specific activity, and people didn't really "know they needed" to have multiple applications running at the same time.

Tog's paradox seems to explain this evolution really well.