Using * to enforce the need to update both tables together
I'm not entirely sure what he is trying to do either. This query is inserting into MAIN rows from IPR_CIP that don't already exist in MAIN. It depends on the two tables always having the same number of columns and corresponding columns having compatible data types. It would be much better to explicitly list the columns you are updating in MAIN and the columns you are selecting from IPR_CIP. The subquery is redundant and can be moved up into the parent. Here's the TSQL code I would use.

INSERT INTO MAIN (col1, col2, col3, ... , coln)
SELECT (col1, col2, col3, ..., coln)
FROM IPR_CIP
LEFT OUTER JOIN MAIN On IPR_CIP.CASE_NUM = MAIN.CASE_NUM
WHERE MAIN.CASE_NUM IS NULL