I think this is a fair question, and I find some of the explanations here wanting. You could implement the same UI the the same external specifications with a

I think this is a fair question, and I find some of the explanations here wanting. You could implement the same UI the the same external specifications with a number of different data models. In theory, your users would never be able to tell. But in practice, a good data model is going to lend itself to performant software that can be changed predictably and without too many bugs. The data model is internal quality.