It doesn't combine them (because different people are subbed to different communities, although what you bring up has been discussed on the github for a while). The biggest thing though is discoverability, where your post is shown in multiple communities easily. Another added benefit is that there are less duplicated posts, your app/device can see that it was crossposted and show you only one in your feed rather than 3,5, 10 duplicate posts (and then you can tab between them)