Use of file names ending with a period

Why does Unix allow files with a period at the end of the name? Is there any use for this?

example.

I am asking because I have a simple function that echoes the extension of a file.

ext() {
  echo ${1##*.}
}

But knowing it will print nothing if the filename ends in a ., I wondered whether it would be more reliable to write:

ext() {
  extension=${1##*.}
  if [ -z "$extension" ]; then
    echo "$1"
  else
    echo "$extension"
  fi
}

Clearly this depends on what you are trying to accomplish, but if a . at the end of the file name were not allowed, I wouldn't have wondered anything in the first place.