Because some reference styles require it, and publishers -- or
university dissertation committees -- are extremely strict about such
mechanical things.
If you have no contribution to make, and here you don't because Word
has no provision for changing the presentation of footnotes
whatsoever, why don't you make no contribution?
In FrameMaker you can group footnotes into paragraphs (using the run-
in sidehead feature), but even there you can't put two columns of
footnotes under a single broad column of text.
In Word you can format the paragraph mark as Hidden, which will
suppress the next footnote from starting a new line -- but, Suzanne
explained a while ago, Word still allocates as much space as the
separate paragraphs would have taken up. (If you don't have many, that
might not be entirely unacceptable. But with 27, no way. Will they let
you do endnotes?)