Common Table Expression, why semicolon?

Usually in SQL Server Common Table Expression clause there is semicolon in front of the statement, like this:

;WITH OrderedOrders AS --semicolon here
(
    SELECT SalesOrderID, OrderDate,
    ROW_NUMBER() OVER (ORDER BY OrderDate) AS 'RowNumber'
    FROM Sales.SalesOrderHeader 
) 
SELECT * 
FROM OrderedOrders 
WHERE RowNumber BETWEEN 50 AND 60

Why?

• To avoid ambiguity because WITH can be used elsewhere
  ..FROM..WITH (NOLOCK)..
RESTORE..WITH MOVE..
• It's optional to terminate statements with ; in SQL Server

Put together, the previous statement must be terminated before a WITH/CTE. To avoid errors, most folk use ;WITH because we don't know what is before the CTE

So

DECLARE @foo int;

WITH OrderedOrders AS
(
    SELECT SalesOrderID, OrderDate,
...;

is the same as

DECLARE @foo int

;WITH OrderedOrders AS
(
    SELECT SalesOrderID, OrderDate,
...;

The MERGE command has a similar requirement.