One of the most unique features is that, even with the free version, you (or other people) can manage events from both the frontend and backend. Most other plugins only work from the backend, at least in their free versions. This opens up a lot of flexibility, especially around any type of event directory site. The ability to add your own custom fields also makes it very easy to customize WP Event Manager to your needs, even in the free version.
Overall, that’s most of what you can do with the free version, which is already quite flexible. But with the Pro add-ons, you can unlock a lot more features.