Storing them in Taiwan makes them immediately available in case China launches an attack. These very important items permit an aircraft to make check whether another aircraft detected on radar is friendly or hostile through coded electronic signals. This equipment is not controversial, but it is a challenge to master flying at night using night visions goggles. These missiles can be used to defend Taiwan's ships from Chinese attack boats, frigates and destroyers. There is a threat to Taiwan of blockade from China.
Submarines are an effective weapons against blockading ships.
However, the US has not manufactured diesel submarines for over 40 years. A consortium of Dutch, German and US firms has offered to build them for Taiwan at an US shipyard, but I do not think there is a strong chance that this will be approved. Countries that manufacture diesel submarines would be better places for Taiwan to make such a purchase. Taiwan's defense strategy calls for stopping an invasion before it reaches the shore. Taiwan's forces need to attack amphibious tanks and landing craft as they approach the island.