Actually, the first one does need the g, should be

  mv topo{log,graph}ical.data

It works because it expands to

  mv topological.data topographical.data

try it with echo instead of mv to see how it expands. Other uses:

  mv myfile.txt{,.tmp}   # add extension
  mv mymusic.{mp3,mp4}   # change extension
  mv myarchive.{zip,jar}