It was a mistake. But the aims was to differentiate  between undefined variable and variable set to empty string.

```
$ if [ -z $FOO ]; then echo "undefined FOO"; fi
undefined FOO

$ FOO=""
$ if [ -z $FOO ]; then echo "undefined FOO"; fi
undefined FOO

$ FOO="a"

$ if [ -z $FOO ]; then echo "undefined FOO"; fi
```
versus

```
$ if [ -z "${BAR+_} " ]; then echo "undefined BAR"; fi
undefined BAR

$ BAR=""
$ if [ -z "${BAR+_} " ]; then echo "undefined BAR"; fi

$ BAR="a"
$ if [ -z "${BAR+_}" ]; then echo "undefined BAR"; fi
```