I thought 'Hi there' was only used to greet people who you didn't know their names, such as you write a complaint letter to customer service. So I feel a little bit awkward when I received messages with 'Hi there' + some random chat/information from my close friends. I have encountered twice such case. Not sure if I have been overreacting on these.
"There" suggests a physical space/place.
You are at your desk. You're busy working and then look up and see a friend or acquaintance whom you only just noticed or were not expecting. You say: "Hi there".
It is informal. It does not mean you don't know the person. In fact, you might know them very well. Or you are at your desk at home, and your child comes up unexpectedly, and you say: "Hi there."
It is not appropriate for writing emails to customer service. So, feeling uncomfortable receiving an email with that great as a customer service person is completely understandable...
There is also an element of surprise. You were not expecting the person to show up unexpectedly and it pleases you.