When you sometimes hand over your phone to show something to your friend/partner/co-worker or let your kid/nephew/niece play a game, the last thing you need is them opening or even worse: touching your important and confidential work emails (whether it’s intentional or them just being clumsy).
Like most messaging and 3rd party email apps nowadays, it should be possible to lock Missive with a passcode, PIN and/or fingerprint.