Why is the buy 1 give 1 model an appealing CSR tool?

The Buy 1 Give 1 is a charity-based business model that integrates the donation of something to someone in need for each purchase made.  TOMS shoes, for example, donates a pair of shoes to a child in need for each pair of shoes purchased from them; hence one-for-one.  While TOMS is perhaps the most well known proponent of this model, firms worldwide are following suit.