I'm just trying to understand, why be at work on the weekend or after hours unless you need/want to get something done? (Man, I wish HN had a private messaging

I'm just trying to understand, why be at work on the weekend or after hours unless you need/want to get something done? (Man, I wish HN had a private messaging system, because I usually have questions that interest me but aren't really appropriate to ask publicly. Thinking of writing one...)