Why do people put full/mostly full cups of coffee in the sink without emptying it out?

One thing I’ve never understood is when people set down a full or mostly full cup of coffee in the sink. Like, do you want it? Why didn’t you dump it out? Are you coming back for it? What’s the protocol here? I never know how to navigate that. I think I find it perplexing that it’s mostly full…not dumped…in the sink…three things that counter each other.