Steps to reproduce:
1. Open Impress. Create a new blank presentation.
2. Scroll up or down with your mouse wheel or touchpad.

What's the point of Impress having such large useless area?

How many times have you drop an element there, outside the slide? (Have you ever done that?)

How many times have you groan because you accidentally scrolled (or maybe you naively wanted to switch to prev/next slide), and instead you got your slide moving from the screen, letting you see a blank totally useless area instead?

MS Office solves this very easily. It just give you a small gap between the slide and the rest of the UI. If you drop things outside the slide, you can scroll there. If not, you can't, because you would never want to do that.