Comment by TeMPOraL
Or just trying to trap you into a synchronous interaction on their terms, pressuring you to give them your full attention and respond immediately.
They send their "Hi", and go do other stuff. You eventually respond with "hi", and they immediately reply with the request. At this point, they know you're around and saw their message - you just replied to their "hi". And you know they know, and also they know you know they know, which was the entire point.
They got to ask you the thing directly, so ignoring it now feel like walking away, which is rude.
That kind of thing tends not to work on me, because if I do reply "hi", I usually then get distracted, and don't reply to the next message for many hours. So they quickly learn it's better to just ask the question straight up.