Well ok ... If a full width newspaper needs the whole sheet and you put columns to reduce the necessary whitespace, you would end with blank space that you can fill with more articles or ads.

You don't really want to reduce the number of sheets for your newspaper because size of the newspaper is an indicator of quality. But trimming off maybe 10% of a dictionary is worthwhile. Size of a dictionary is a quality indicator too, but the absolute production cost is more for a book so saving production costs is more pressing.

That's easy. The columns have to be narrow enough that you can still read a full column of scripture while your éclair rests on the other half of the page. That's why a full-spread layout is referred to a "qautréclair."