Cenvat credit - essence of tpt copy

Respected sirs, if we buy any material n if we are eligible take cenvat credit on excise duty paid,normally we are asking the supplier to provide Transporter's Copy stating ED details..why we need to get TPT Copy..are we not eligible for CENVAT credit if TPT copy not provided..?? however ED details are available in invoice,then again wt is the essence of TPT Copy again..?? Kindly clarify...?