I use it all the time. The easiest example is when I am developing something and am logged in as Admin into web interface and then want to check how it works as a non-logged-in user. Or as multiple sequential logins. Sometimes it is faster to close and reopen incognito than logout/re-login.

Or check the newspaper article I don't want linked into whatever is normally tracking me.

Ironically I use it as a Chromium/Chrome developer to check whether certain documents are visible to users outside of Google, as a way to determine what information is internal-only.