[okusername]

People do this because they want to know that you're available for the conversation. Like ringing the phone.

[mrunkel]

No. They do this because they know if they just ask their question you'll ignore it because it's not important.

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.

[dennisgorelik]

Are you, personally, saying "Hi" because you think that if you ask your question -- then your message recipient will ignore you?

I am saying "Hi" (without anything else) - to check if person is available for discussion (exactly like okusername described it).

[RHSeeger]

No, "they" don't. Some people do, sure. But some people are doing as the parent mentioned; ringing the phone. Letting you know they need to have a discussion with you, so you can reply when you're ready.

[nowandlater]

Pretty true. The worst is when they butter you up a bit with, "how was your weekend?", or "The weather is pretty nice today". Like they actually care about you, ha!