user: Question: What is the main use for the structure listed in Walker City, Iowa?
Columns: ['Pinnacle height', 'Year', 'Structure type', 'Main use', 'Country', 'Town']
Types: ['text', 'real', 'text', 'text', 'text', 'text']
Sample row: ['m (ft)', 1969.0, 'Guyed mast', 'VHF-UHF transmission', 'U.S.', 'Hemingford, Nebraska']
SQL:

assistant: SELECT "Main use" FROM "Table" WHERE "Town" = 'Walker City, Iowa';