Is the bank verification letter necessary ? what is its format?

I am sending my parents to US where my sister is working as a doctor.I have filled the DS-160 FORM,Submitted the documents to VFS,Fixed a joint appointment for both of them which would be held on 4th of September’2012.
They would be spending on their own and are not sponsored by my sister.
I wanted to ask that :

1)Do my mom and dad require a letter to us consulate genrale asking for the grant of B2 visa as listed under the documents section of
2)I have the bank statements for both fo them.Then why is the bank verification letter necessary? please help me if anyone has its format?(as I was asked by my bankers)

thanking in anticipation of prompt reply…