What does mean by " I PROMISE TO PAY THE BEARER THE SUM OF.........RUPEES?

10 Rupee note - Reserve Bank Of India
October 1, 2008 3:17pm CST
I think everybody will have seen 10, 20, 50,....... Rupees note. In 10 Rupees note, a sentence is written " I PROMISE TO PAY THE BEARER OF THE SUM OF TEN RUPEES". Like wise in every rupee note this sentence is written. Can anybody tell what is the purpose of written this sentence.
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