Make non-breaking spaces visible in the editor. This may be a prerequisite to a VE feature for inserting / modifying non-breaking spaces. Entering non-breaking spaces easily, while common on some systems (OS X), only exists in full-blown word processing applications (Word) on others. It may not be obvious to the user that it is possible to insert non-breaking spaces in VE.
LibreOffice displays them with a grey background, always:
Word displays them only when non-printing characters display is turned on, as ° (normal spaces displayed as ·):