Cashflow

In Cashflow, direct method,
To find Cash paid to supplier, we simply make T-Account of Creditor and find Cash paid. In that, we put Cost of Sales instead of Purchases. Why is that so? Like if we have Cost of Sales of 100000 so we simply add Opening Trade Payable +100000-Closing Payable = Cash paid to supplier. I have attempted various questions and each of them use Cost of Sales instead of purchases even we are given Balance sheet of two years so can find purchases.