As another one who uses this feature often, I can remember how I discovered it a long time ago: accidentally pressing it once on a page and thinking "oh, that's useful", then regularly using it after that.

On the other hand, I'd rather Alt+Left be disabled inside text inputs, as I have quite a few times accidentally went back while trying to move the cursor.

The odd thing is that this shortcut is not mentioned in the Back button's tooltip of neither Firefox, IE, Opera, nor Chrome.