Putting a question mark at the end like this imitates the change of intonation when speaking with uncertainty. Eg: I think I left the phone in my car. Vs I

Putting a question mark at the end like this imitates the change of intonation when speaking with uncertainty. Eg: I think I left the phone in my car. Vs I think I left the phone in my car? While technically incorrect in written English, this is very common in casual speaking and has found its way into casual writing.