Basically for cleanup. There are various types of questions (e.g. spam, questions that are clearly not about mathematics) that just don't belong here, but waiting for five people to close them is just wasting the time of four people. Some of these questions most people never see because the moderators also delete them, which is exactly the idea; they shouldn't be cluttering up the site.
When a moderator votes to close as off-topic they also have the option of migrating the question to any other SE site. As I understand it, non-moderators can suggest such a migration, but it is quicker and easier just to have a moderator do it.