Danish union vs A-kasse: What's the difference?

When you first arrive in Denmark to work or look for work, the last thing you need is another monthly expense. So many foreigners "save money" by not joining a union. And I was one of them. To be honest, joining a union never even occurred to me. In the US, unions are either for hands-on workers – steelworkers, hotel maids – or for civil servants, like schoolteachers and cops. Knowledge workers and creative types are almost never unionized. But that's not true in Denmark, where engineers, doctors, lawyers, bankers, managers, and writers regularly join unions. Unions can arguably be even more important for foreign employees than they are for Danes.