[ohazi]

If you trust the recipient, but there's a chance that they will be threatened and asked/forced to reveal the message that you sent them, they may appreciate a mechanism that gives them plausible deniability.

Obviously a recipient could make a copy of any disappearing message that they receive if they really wanted to, so you do need to actually trust them. It's less for your security, and more for theirs.

[senectus1]

yup, also you can trust the person but maybe not trust their long term security. the short the lifespan the small surface of attack.

[MrGilbert]

Depends on "they", I guess. Plausible deniability goes south, once you are greeted with a wrench. Reminds me of https://xkcd.com/538/