Floating Or Flat Mockup?

While Designing a mockup for an App or a concept, I find it difficult to choose between Flat Mockup and Floating Mockup.

I'm interested to know why Floating Mockup gives a more 3-dimensional look and good aesthetics than a Flat Mockup which takes up the same 3-Dimensional vector space.

Floating Mockups widely used across various product landing pages (samsung, Apple, Microsoft), Dribbble and Behance.