A sea mine is like a land mine in that you always have a safe path through that you know and can communicate to allies. Even if enemies know the location of the minefields, you’re still forcing them to go through an area of your choosing rather than allowing them free movement. Most likely, Iran will have mined the areas closer to the UAE side of the strait, forcing any ships to hug Iran’s coastline and allowing them to strike with even their shortest range missiles.