Alright, here's an example:

  touch 'file with spaces'
  for file in $(ls); do
    rm -- "$file"
  done

This will not work, because $(ls) will get expanded to three items, not one. rm will be called on the files "file", "with", and "spaces"...which is not what was intended.

Now do this:

  touch 'file with spaces'
  for file in *; do
    rm -- "$file"
  done

This works correctly.