What is the main use for the structure listed in walker city, iowa?
CREATE TABLE "structures_past_or_present_between_550_a" (
  "pinnacle_height" text,
  "year" real,
  "structure_type" text,
  "main_use" text,
  "country" text,
  "town" text
);
SELECT "main_use" FROM "structures_past_or_present_between_550_a" WHERE "town"='walker city, iowa';