How to set a variable in a Dynamic SQL query

Here is the scenario, You have a stored procedure that builds dynamic insert SQL which means the "INSERT" statement is being composed on the fly based on the table name but at the same time you need to get SCOPE_IDENTITY or the last identity number inserted for further processing (e.g History table)

	DECLARE @nvcSQL		NVARCHAR(MAX)
	DECLARE @pvcTableName	VARCHAR(255)
	DECLARE @pvcColumn	VARCHAR(255)
	DECLARE @pvcValue	NVARCHAR(MAX)

	--This is used to actually get the primary key for identity record
	DECLARE @dvcPrimaryKey VARCHAR(255)
	SET @dvcPrimaryKey = ''

	DECLARE @dintPrimaryKey INT
	SELECT @dvcPrimaryKey = ISNULL(NAME,'') FROM SYS.COLUMNS WHERE OBJECT_NAME(OBJECT_ID) = @pvcTableName AND Is_Identity = 1 

	-- Only execute when there is a @pvcValue.
	IF @pvcValue <> ''
	BEGIN
		SELECT @nvcSQL = 'INSERT INTO ' + @pvcTableName + '(' + @pvcColumn+ ') VALUES (' + @pvcValue + ')'
		SELECT @nvcSQL = @nvcSQL + ';' + 'SELECT @dintPrimaryKey = SCOPE_IDENTITY()'
		--PRINT @nvcSQL

		EXEC sp_executesql @query = @nvcSQL, @params = N'@dintPrimaryKey INT OUTPUT', @dintPrimaryKey = @dintPrimaryKey OUTPUT
		--EXEC (@nvcSQL)
	END

	IF @dvcPrimaryKey <> ''
	BEGIN
		SELECT (@dvcPrimaryKey + ' = ' + CAST(@dintPrimaryKey AS VARCHAR(10)))  AS PrimaryKey
	END
	ELSE
	BEGIN
		SELECT '' AS PrimaryKey
	END

Question: Why do we need to use sp_executesql instead of EXEC? Because EXEC just execute a SQL command without returning/expecting any result from it.