These subs aren't meant to threaten Israel. They have the potential to deal some damage to a surface task force in the Gulf, but more importantly they pose a threat to what the Americans refer to as Sea Lines of Communication, or in other words, the oil supply line. If Iran mines the Gulf and targets shipping, you can say goodbye to any global economic recovery.
Netanyahu toured Gaza border with Ya'alon, Eisenkot and southern command chief (Haaretz)
from the article: Iran Navy acquires new submarines amidst nuclear tension