The progress bar makes waiting more exciting… and mitigates our fear of death. Journalist Daniel Engber explores how it came into existence.

So if you have a progress bar that just moves at a constant rate – let’s say, that’s really what’s happening in the computer – that will feel to people like it’s slowing down. We get bored. Well, now you can start trying to enhance it and make it appear to move more quickly than it really is, make it move faster at the beginning, like a burst of speed. That’s exciting, people feel like, “Oh! Something’s really happening!” Then you can move back into a more naturalistic growth of the progress bar as you go along. You’re assuming that people are focusing on the passage of time – they’re trying to watch grass grow, they’re trying to watch a pot of water, waiting for it to boil, and you’re just trying to make that less boring, less painful and less frustrating than it was before.

https://www.ted.com/talks/daniel_engber_how_the_progress_bar_keeps_you_sane