Since on most platforms the tabs go all the way to top of the window now, there isn't room to grab the window to move it around. Those spaces act as extra drag targets.
Bonus: You can resize any window on these same distros by 'Alt + Right Click'. It will allow you to resize along the closest border(s).
As many users rely on the functionality, they had to be added back.
You can configure them in Customize → Drag Space (on/off)