Chris M. answered 11/09/21
BA in International Studies; M.B.A.; 8.5 years studying for PhD.
I was going to let an intenational relations major answer this, but I see that there haven't been any answers yet, so I will give you an answer.
EEZs extend 200 nautical miles (which are roughly 1.15 statute miles long) from the coast of a country. As the Spratlys are spread out over 158,000 square (statute) miles, the area covered is very substantial. The Spratly's are not only important for their economic value and the minerals under the sea floor. They are also very important in terms of the security they can provide (including the security of shipping lanes) and for the national pride--as well as sovereign integrity--of a country. Coordinated fishing boats can serve as eyes and ears of a country as well as an unofficial navy. If your country can argue that it is protecting its people's historical territory and their birthright, it helps the government appear more legitimate and it keeps the focus of its people on how foreign countries are threats instead of short comings and aggressions of its own government.
I hope someone with an international relations background can add to this explanation.