Why do people say, "Allah hears the one who praises Him - Our Lord, and to You be all praise"?

It is prescribed for the Imam to say "Sami' Allahu liman hamidah" (Allah hears the one who praises Him), and it is prescribed for the congregant to say "Allahumma Rabbana wa lakal-hamd" (O Allah, our Lord, and to You be all praise). This is based on the Prophet's (peace and blessings be upon him) saying: "When the Imam says 'Sami' Allahu liman hamidah,' then say 'Allahumma Rabbana wa lakal-hamd,' for indeed, whoever's saying coincides with the saying of the angels, his past sins will be forgiven." The meaning of "Sami' Allahu liman hamidah" is: Allah responds to the one who praises Him.