They're also good when you need a reset button (though it should be called Reset).
For example, at a vehicle insurance site I wanted to see quotes for different things: a brand new scooter, one a few years old and a rusted beater, for two different drivers (so different postal codes, different damage-free years).
Unfortunately the site did not have a Reset button, and it stubbornly kept license plate info of the first vehicle (which was greyed out and unchangeable, as well as related buttons who don't get filled in if a license plate number is). I ended up opening another browser to look up the others. I also could have cleared my browser cache, but was in the middle of something on Github and did not want to clear my cache right then.
Sometimes one computer may be inputting for multiple people, so while you usually want to preserve inputted data when users go back to a form or hit the back button, if it really is a whole new user then offer a manual method of clearing that data if that makes sense for the form.