Using comma separated value parameter strings in SQL IN clauses

CREATE FUNCTION [dbo].[CSVToTable] (@InStr VARCHAR(MAX))
RETURNS @TempTab TABLE
(id int not null)
AS
BEGIN
;-- Ensure input ends with comma
SET @InStr = REPLACE(@InStr + ',', ',,', ',')
DECLARE @SP INT
DECLARE @VALUE VARCHAR(1000)
WHILE PATINDEX('%,%', @INSTR ) <> 0
BEGIN
SELECT @SP = PATINDEX('%,%',@INSTR)
SELECT @VALUE = LEFT(@INSTR , @SP - 1)
SELECT @INSTR = STUFF(@INSTR, 1, @SP, '')
INSERT INTO @TempTab(id) VALUES (@VALUE)
END
RETURN
END

DECLARE @LIST VARCHAR(200)
SET @LIST = '1,3'
SELECT Id, Descr FROM CSVDemo WHERE Id IN (SELECT * FROM dbo.CSVToTable(@LIST))