Sometimes it does not make sense to display certain custom object fields, unless others have been checked.
And sometimes it might be useful to require the input for certain custom object fields.

A use case would be a license custom object - certain licenses for hardware products, might require a hardware code, while other licenses for software products might not.

Being able to set conditions upon which certain custom object fields are displayed would be ideal here.

A more extensive functionality that would be incredibly powerful, but probably also a lot more difficult to implement, would be conditions based on Lookup relations:

For example a license custom object might have a lookup relationship linking it to a country custom object.
If an admin were now be able to define conditions in the license custom object, which are checked against the country custom object and its fields, this would allow for immensely powerful functionality that would allow Zendesk Admins to set up dynamic relational data models.
E.g. "if the region field of the related country custom object is Europe or null (if there is no related field) -> require a custom object field to be displayed / filled.