This is needed because some Python extensions require strict versioned depends on python-numpy, and using this helper script is the easiest way to get them.
The helper script uses the information stored in /usr/share/python/dist/python-numpy to generate the Depends information; that file is also used by dh_python2 (and from dh_pysupport when implemented) to generate the same set of Depends. This script allows you to not use dh_python2 if you don't want to.
This program is not a part of debhelper (and it's provided by python-numpy).