It is said that the elevated radials are an artificial ground plane,
simulate a ground plane, form a virtual ground plane, or approximate a
ground plane. It may be that the radials perform the same function as a
ground plane, but do not form a real ground plane.

For example, the Butternut antenna manual at following web address says that
elevated radials are an artificaial ground plane.

http://www.bencher.com/pdfs/00361ZZV.pdf

What functions do the radials perform that are the same as a real ground
plane? What similarities are there between radials and a real ground plane?