WELCOME!
I think most people would agree that carrying it with you all day is not really an option. It might not seem heavy, but give it a few hours and you'll probably be pretty miserable.
People do different things. Lots of times the hostel will have lockers and you can put it, or at least anything valuable in a locker. I've seen people that lock it to their beds. I don't do either of these things; I just don't bring along ANYTHING valuable. I keep my credit card, passport, and cash on me at ALL times. So the only thing in my pack that is left during the day is clothing and toiletries....b/c if someone wants to steal that stuff, well ok, what am I doing to do, and it's not too expensive to replace.