As best I can tell, there exists a feature in Writer that allows user-defined fields to be created and inserted, but Impress doesn't support anything like that feature. I would like to have this feature added to Impress.
The use case I'm thinking of is: In a large-ish presentation one wants to turn into a template, a variable could be used to unify dynamic text on the title page text and some other pages where much of the intervening pages are static. For example, you have an interdisciplinary process and want to tailor your presentation to a specific audience, but you need to update text in a few palaces in-line, so to speak, not merely on the title page and backgrounds as could be achieved with two user-defined master pages.
Does this make sense?
Thanks for filing the idea. We know this already in another bug.
*** This bug has been marked as a duplicate of bug 53548 ***