why does the RAF think it needs IEng/ CEng if these can be bought on promotion to the relevant rank anyway?
The Armed Forces has always been very good at training people. What it wasn't good at for a long time was ensuring that training could be equated outside the RAF. Now most training is aligned to meet the criteria set by the relevant professional bodies (not just engineers), your rank is an accepted break point for a level of experience.
If you've never worked outside the RAF or in DE&S (in recent years), then you probably don't see the value. Or you are one of the aloof old guard for whom "Don't you know who I am?" suffices when your professional ability is remotely questioned.