I dont know anyone who uses it as a routine way to text. A couple commenters said it’s popular for some people that way, though, so maybe it is. I guess it allows for more natural conversation if you are talking like you aren’t making a permanent record.
If you don’t use it, it doesn’t seem that useful, but it actually offers a pretty good utility. There are a lot of situations where you want to show something to someone, but you don’t want/need to permanently have a picture of it in your phone. Just looking at a text conversation with a friend who doesn’t have it, I see pictures we’ve sent back and forth with screenshots of restaurant reservation times and movie show times that have already passed. There’s things they’ve seen at the store that they wanted to know if they should buy for me. That kind of stuff doesn’t need to be permanently stored on my phone, but it is. Yeah, both of us could go in and delete those pictures, but realistically, we won’t.