Often people are looking for some sort of validation, even when it's not obvious.
If they say "I can't join you in the bar today, I have too much work to catch up on", "K." is not a good answer. Several aspects needs to be addressed, ideally:
That's too bad
Next time
Commentary on the state of work: Keep your head above water/your boss is such a jerk/we'll make up for it after your deadline on Thursday/whatever, depending on the situation and your relationship.
Basically, it's a way to show that you care about what they're telling you. It can be a bit exhausting at times.