What does there signify in expressions like "Dude are you out there?" or "I am there for you"? It is not referring to any previously mentioned location, right?
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If I heard or read
I'd assume it was asking someone whether he was outside, so it doesn't necessarily relate to a previously mentioned location, but it does refer to a general location (e.g., outside my house).
If someone says or writes
I would understand it to mean that the speaker/writer is saying that he or she is always willing to help me if I need help. I say this to my son to let him know that I love him and will always be ready to listen to him if he needs to talk about his problems, to support him if he has financial problems, to do whatever I can do to help him if he ever gets in trouble.
The meaning has nothing to do with a location, but is similar to what military people and police mean when they say "I've got your back": "I'll make sure nobody shoots you in the back".
Let me differ with Bill Frank. I think "there" works as an adverb of place, an indexical, which points—yes—to a place. But the place at hand can be physical or non-physical, i.e., metaphorical. When I say:
I mean that wherever you need me (in whatsoever place or whatsoever situation) I will be there.