The Pro versions help maintain the developers, the Zorin Brothers, and help towards development of the next release. The pro versions contain a lot more apps, some of which you may not need. I did post details of what is included in Zorin 15 Pro awhile back so suspect will contain the same additional apps.
However this is an old thread. You can add some of the apps themselves but with the premium edition you know it will work out of the box.