What is the significance of oauth_token_secret returned by POST oauth/request_token


As I understand, the oauth_token_secret value is used as the signing key (consumer secret + & + token secret) during signature generation. But while making call to POST oauth/access_token, even if I don’t include oauth_token_secret (obtained from POST oauth/request_token) in the signing key, I am able to obtain access token and access token secret successfully. Then whats the use of oauth_token_secret obtained from POST oauth/request_token.