A sign points to something. As an aside, this is why we baptize infants. The answer is that it is for the same reason that Abraham was instructed to circumcise both his children and those living within his household. Paul contrasts the promise to Abraham with the Law. Verse 14 says that, if you can get anything from the Law, then the promise which God made to Abraham was a lie.
Law Brings about Wrath.
For the promise to Abraham or to his descendants that he would be heir of the world was not through the Law, but through the righteousness of faith. The Law can't do that. The Law can only condemn. It can only pronounce a person guilty. Any attempt to bring the Law into salvation results in canceling out faith.