Good question. Here, "please" it is is a reduction of the phrase/question "[would you] please tell me ___" You should never use the plain question on the phone with a stranger, it will sound rude. With a friend, or even someone you've been talking to for a minute, the plain form is usually OK. English lacks a formal/informal distinction in verbs & pronouns, so we have to add extra words to show politeness or respect, where in Russian you'd use a form of ты/вы.

If you're using "please" in this sentence you're being polite, otherwise you're kinda implying that the person has the obligation to tell you where John is

You can ask this question with or without using "please" and it conveys the same message. One is more polite.

Kак разъяснется ниже, «please» является сокращением более полной мысли. Например, "Can you please tell me if John is home?"