Sending a submarine to West Asia and publicly announcing it only has domestic use. After the embarrassing performance of US military in Syria after 6+ US bases were attacked by 100+ shells, rockets and missiles some days ago and it made a lot of sound in US media they had to pretend to be doing something and not look weak in the American public's eye. They couldn't do anything real since they knew any respond to those attacks would bring another attack that time with 500+ missiles, etc.

Otherwise these submarines have to be within ~1500 km range which means in Persian Gulf that has a shallow depth in most places and it is not a place to use a nuclear submarine. Not to mention Iran's dominance over Persian Gulf which makes it completely ineffective