Why do people step back three steps before the Amidah (as mentioned [here][1] in Mishnah Berurah, S'if Katan Gimmel) if the Gemarah only mentions stepping three steps **forward**? Is there a kabbalistic source or is it just practical and not really a requirement (as the Mishnah Berurah quotes from E.R.)?


  [1]: http://hebrewbooks.org/pdfpager.aspx?req=14170&st=&pgnum=254