It is not really a Mendix question, but i will have a go anyway.
Defining “done” is an opportunity to make sure you all have the same understanding of what is required. And an opportunity to rethink if nothing was forgotten.
Not having a clear and shared understanding of requirements is one of the main problems in IT, so getting this more clear is very valuable.
Personally i prefer having really small feedback cycles (inspaect and adapt) over long discussions on the definition of done, by the way. You can always sharpen the definition as you go.
I hope this helps.
So that all parties involved know what it means for something to be done