Thats easy - it controls the sea lanes coming out of the Red Sea. They could interdict US assets moving through
the Suez Canal, shipping moving into/out of Eilat, West Coast of Saudi and the port of Aquba. They could place AA missiles there which would make Israeli flights over the area impossible (how they got to Entebbe for the raid).
This is a very provacative move.
Remember how the "last straw" in the June 1967 "Six Days War" was the bottling up of the Gulf of Aqaba/Eilat by the Egyptians at Sharm-el-Sheikh?