One reason is you're paying attention to something else, like riding, the other riders around you, cars, etc., rather than constantly looking down at your drivetrain. IME, with more modern drivetrains (10 speeds and up) it's easy to find yourself way up the cassette in the big ring or way down the cassette in the small because it is just so darn smooth. It's also because there's a lot of overlap, of course, but I find I don't mind that. So the chain rubbing on the big ring when in the small ring and the third smallest cog is a handy reminder.

Older drive trains (6-8 speeds, DT or STI), I find myself much more aware of which ring I'm in because I'm always about to run out of low gears.