You live in a tent with at least one other person which means that your personal space (when you're sleeping, showering or even peeing) has been somewhat demolished. You spend every day drinking beer - the primary activity of the warm-up days, and perhaps the reason why they are so popular with the younger festival guests. And all the while, you have a certain feeling of anticipation for what the real festival days will bring - the best is yet to come.
But other than that, life goes on in the slum as it would in any other place. There are triumphs and defeats, happy moments and tragedies, ups and downs. Among the good things is the exceptionally low rate of violence. When you cram 80,000 people into tents and feed them alcohol all day, you might expect that there would be bunches of fights, sexual crimes and theft. And while it is not uncommon to have your camping gear "borrowed" by someone during the warm-up days, it really is remarkable that the crime rate is so low, beating many other major European festivals by miles.
So there you have it: I think the warm-up days of Roskilde Festival (and any festival, really) have a special appeal to people, because it's a way of escaping your everyday life... By living your everyday life in a different setting. People are creatures of habit, and whether you enjoy it in a dirty field outside a small town in Denmark or you do it at home in your usual setting, the appeal of a cold beer in the sun is hard to deny.