is the same thing as . If you delete them all, the vertical spacing they represent in your document will be lost (not that I recommend using them for vertical spacing).
It's not a question of "asking" for them. If you hit the return key in Book View to add vertical spacing, Sigil automatically generates the code:
in code view.
Prior to v0.7.4 the same action would have created:
There is no fundamental difference between the two when it comes to rendering.
Also: any unicode non-breaking space characters in your source document will be converted to   when opened in Sigil. Any horizontal spacing that may have been achieved with consecutive non-breaking space characters will be lost if you delete all the  s