When I'm asking this to the junior engineers on the team I'm usually trying to tease out what they think is fun or what their non-work interests are. I have many of them that are over engaging at work (they're achievers) or having trouble adjusting to a new city without as many of the forced social situations of college. Me asking this is to get to know more about you so I can a) talk with you about non-work things, b) learn a bit more about you, c) offer advice when I see self imposed burn out approaching.

If all you do is work work then I think you should probbably re-adjust a bit on the weekends or you'll narrow focus too or you're just working too hard. If you're doing side projects, I'm an engineer too, I LOVE to hear about that stuff. It's why I read hackaday and hacker news.