different usage of SQL's stored functions and stored procedures

gistfile1.sql

-- schema for exchange:
CREATE TABLE exchange(cur VARCHAR(3), rate DECIMAL(12,3));
INSERT INTO exchange VALUES ("eur", 1.00), ("usd", 1.28), ("czk", 25.98); 

-- stored procedure
DELIMITER :-)
CREATE PROCEDURE exrate(IN cur_in DECIMAL(12,3), IN currency VARCHAR(3), OUT cur_out DECIMAL(12,3))
BEGIN
  DECLARE course DECIMAL(12,3) DEFAULT 1;
  SET course = (SELECT rate FROM exchange WHERE cur = currency);
  SET cur_out = cur_in * course;
END:-)
DELIMITER ;

-- usage:
SET @FOO = 0;
CALL exrate(2.30, "usd", @FOO);
SELECT @FOO;

--################

-- stored function
DELIMITER :-)
CREATE FUNCTION exrate2(cur_in DECIMAL(12,3), currency VARCHAR(3)) RETURNS DECIMAL(12,3)
BEGIN
  RETURN (SELECT rate FROM exchange WHERE cur = currency) * cur_in;
END:-)
DELIMITER ;

-- usage:
SELECT exrate(2.30, "usd");