The difference is that if you want to have the conversation in real time you want to see if they're there first. It's the the same difference between leaving a voice mail and repeatedly calling.
But it doesn't really tell you if I'm available to chat -- my availability depends on context.
If a coworker texts me "Hi" at 7pm, I'm probably not going to respond until work hours. If he texts me "Hi, haven't spoken in a while, let's catch up", then I probably will respond.
Or, if I'm in a meeting and someone texts "Hi", I'm probably not going to answer since I don't know if they have a quick question or want a 30 minute chat, but if they say "Hey, what's the name of that guy in engineering that you said I should see?" then I'll probably just send a quick answer.
