I'm going to lump these together because the answers are really quite similar despite the stretch in getting there. Let's call it poetic license, shall we.
I can't resist the "Captain Obvious" response - "On baby's bottom." But seriously, you can use a fitted anywhere you'd like. In fact, they can be quite convenient on the go because you've already got a cover going. Might as well keep slapping a fitted underneath it.
And when?...The fitted is a great option for nighttime use. In fact, I was searching for a good nighttime solution when I decided to participate in this event (I admit, I was a 'sposie user at night and TERRIFIED to try anything else. After all, the Bug has enough trouble sleeping. I didn't want to add the problem of a wet diaper to the mix.) I was quite excited to try my new fitteds on my very heavy nighttime wetter.