Really - someone explain to me what a preset is? The docs talk about their advantage and how you can import and export them but not what they are for or what you can do with them....
I think it's like a template (in the broadest sense ) for a database instance - ie. it provides you with the fields ready made and any fancy formatting (which I always struggle with) already done. So all you have to do is add the actual entries. (And this is a very good candidate for an FAQ type question in the docs....)
The database preset is a predefined database activity structure (fields, templates but no user data) conveniently archived in a zip file which you can apply in a new database instance as a fast way to create a new activity with that predefined structure. Typically someone builds the first instance of the activity, saves a preset of it and then share it so that others can use it to crate similar activities (see for instance presets in the plugins repo https://moodle.org/plugins/browse.php?id=16&list=category). The database preset does not include user data so if you want to share data as well you need to export it. Then others can take the export file and import it to add data to their instance of the activity.