It's an indicator of Luke’s family’s poverty that Dad seems, on some level, to not want to heat the attic where one of his sons lives. Mother’s dirty looks, though, suggest that she’ll go out of her way to protect Luke’s wellbeing. Discovering the vents is huge for Luke, as it introduces him to the world outside his house. More importantly, the view outside isn’t mediated by his parents, as his books and toys have been. Being able to look outside gives Luke access to a world that’s totally his own—a sign of his growing maturity.