Additional Fields Purpose
The “Additional Fields” functionality was designed as a way for site owners to modify Events with custom fields that would be displayed without the need to modify templates. It was an attempt on our end to minimize users having to perform coding changes to add custom properties.
I could see the potential for these fields to drive the Event Filters and I would highly suggest you speak up in our User Voice for this feature. We actively review features requested and as possible pull them into our release plans.
Please let me know if you have further questions on “Additional Fields” or other functionality.