The US' strategic ambiguity on Taiwan issue has served its purpose in the past decades as the US recognized China as a frenemy, ie. not a friend, but not an enemy. Since Xi came to power, he saw the US as an enemy and openly allied with the US' adversaries such as Russia, N Korea and Iran. The US and China are now strategic rivals.
President Biden's open declaration of defending Taiwan in case China launches an invasion will help maintain peace in the Taiwan Strait and keep Xi in check.