Daniel Engber, writing for The New York Times (via The Tech Block):
“People wait for all sorts of things every day, sometimes more happily than others,” wrote the interface designer Bob Stahl in a 1986 article for Computerworld. “The problem is how the user feels about waiting.” At the time, machines were often slow and unreliable, and users didn’t always know when their programs crashed. A “progress bar” might mitigate frustration, Stahl suggested, by signaling that bits were flipping with a purpose somewhere deep inside the C.P.U.
It’s true. Even now, when computers are a bazillion times faster than they were 30 years ago, it’s calming to have a cue that the computer is working, the website is loading, or the iMessage is sending.
And, so long as we’re talking about progress bars, see also this classic xkcd comic.