The other person isn't necessarily waiting for the response. At the least, its a communication that you are about to ask them something, i.e. check back here in the next minute or so while I type out my question. Also, "Hi" is a good way of finding out if a person is present, sometimes you might ask to call them instead of typing your question. It also might be the case there are multiple people who can answer the qu…
Chat is natively async: you don't need to know if people are there to ask a question, they might answer at any point in the next few minutes or hours. If you do post just "hi" in a message, your question should come within seconds after.
I use IM as more informal, (AND/)OR conversations with likely back-and-forth, that may require the other person to be present to be of use.