People do this because they want to know that you're available for the conversation. Like ringing the phone.
If they say "Hi" and wait for you to respond, now you're engaged and it's harder to ignore after they pose the question.
To carry your analogy further, it's more like calling someone and hanging up before they answer in order to prompt them to call back.
I am saying "Hi" (without anything else) - to check if person is available for discussion (exactly like okusername described it).