Why do banks use intermediary banks?

Agility Bank, National Association 4 B2 Bank, National Association 16 Bank of Billings 3 BANKWELL Bank 10 Exchange Bank of Missouri 6 Farmers Savings Bank 2 Farmers State Bank of Brush 2 FIRST CMRC Bank 8 FIRST National Association National Bank ALASKA 11 First State Bank Southwest 7 Germantown Trust & Savings Bank 6 Hicksville Bank 7 Huron Valley State Bank 3 Landmark Bank 8 MEADOWS Bank 12 NORTH SALEM State Bank 12 Port Washington State Bank 4 Southwest Bank of Weatherford 4 St. Johns Bank & Trust 8 United Minnesota Bank 5