IM is short for Instant Messaging. As the name implies, it is intended for instant gratification. However, even though one might get the message immediately, it does not mean they will reply immediately. For example, when I am not home, I won’t stop to answer every IM message that comes my way. I am also not one of those people blissfully looking at their phone while walking down the sidewalk. This means if I was to answer every IM every time one comes in, I will have to stop walking, move to the side so I am not in anyone’s way – compose my reply and go back to walking. You can see how this could interfere with me getting to where I need to go. With that being said, if I was to reply to IMs while I am out, it will be at places where it would make sense to devote to the IM.