http://www.w3resource.com/mysql/mysql-procedure.php

• Stored procedures are fast. MySQL server takes some advantage of caching, just as prepared statements do. The main speed gain comes from reduction of network traffic. If you have a repetitive task that requires checking, looping, multiple statements, and no user interaction, do it with a single call to a procedure that's stored on the server.
• Stored procedures are portable. When you write your stored procedure in SQL, you know that it will run on every platform that MySQL runs on, without obliging you to install an additional runtime-environment package, or set permissions for program execution in the operating system, or deploy different packages if you have different computer types. That's the advantage of writing in SQL rather than in an external language like Java or C or PHP.
• Stored procedures are always available as 'source code' in the database itself. And it makes sense to link the data with the processes that operate on the data.
• Stored procedures are migratory! MySQL adheres fairly closely to the SQL:2003 standard. Others (DB2, Mimer) also adhere.