When I was a teenager, my bedroom door had a lock on it because it was that way when we moved in, not because we installed it. I only remember locking the door once– when I was crafting birthday presents for my family and didn’t want anyone to walk in, see what I was doing, and ruin the surprise. My mom actually did try to come in while I was working and asked me why it was locked. I explained later when I gave her her gift.
I hear what people are saying about basic consideration and that a closed door should be enough, but honestly, people are people and sometimes it just isn’t enough. People are careless and forget. More than once I have been walked in on while changing by people who just don’t think.