There are a few nouns denoting places where we engage in our regular activities that use this grammar, for example: School, work, home. Since the child is a student at that school, we usually do not use the article: She went / goes to school every day. She is at school. Otherwise, we would use an article: Her parents went to the school to pick her up. Likewise, the home you live in does not require the article: Joe is at home = Joe is at the home where he lives. We went home = We returned to our home. But in reference to someone who doesn’t live there, we need the article (or a possessive): Joe went to the home of his girlfriend’s parents. It’s the same with “work”. “Joe is at work” means that Joe is a his usual workplace. (It can also simply mean that he is working at the moment.)