BEGIN / END are control flow markers in the SQL language. They help to identify the scope of things like variables.
They do not directly impact the calling of any SQL commands. 

BEGIN
{
sql_statement | statment_block
}
END

Example

BEGIN
DECLARE @myval as decimal( 5,2 );
SET @myval = 155.55;
IF (@myval = 155.55)
select 'Yes!';
END