Incognito was never meant to make you think you're hiding anything from the browser that you're typing everything into. It's about loading a website without any prior context (i.e. cookies) and not saving the state when you're done.
Incognito is useful for debugging cookie state issues without having to nuke your existing cache only to find out it wasn't the issue.
It's also useful when you have to log in on someone else' computer. Open incognito, log in, do your business, close the window. No risk of accidentally staying logged in. (But if they're actively trying to mitm you, incognito won't matter. Just don't use any devices that person has access to).