As many have said, it really does depend on how sensitive or fair your skin is. Also, it depends on what your diaper usage is like, do you simply wear, wear and wet or wear, wet and mess? I personally don't think that it is always necessary to powder and cream up, but as an AB, I find that powdering up and putting on some diaper cream beforehand really contributes to that full-fledged baby experience.
My skin tends to be pretty resilient, I usually only get blemishes on my behind if I mess and even then that's rare. I can sit in a wet diaper for literal hours and not get any form of irritation and that is with or without baby powder. Baby powder, however, goes a long way in keeping things fresh down there it also makes you smell like you just got of the nursery
Diaper Cream has been becoming a must for me, if I coat my bottom in some diaper cream or zinc cream before taping up I find the amount of irritation I get, if any, as previously stated it's rare that I get irritated during my diaper-time, is nominal. Diaper Cream also keeps messes from sticking to your bottom and can make clean up a fair bit easier. I would say that a conservative slathering of diaper cream beforehand minimizes the risk of blemishes and irritations, so it can be a good preventative measure for anyone who has had those problems in the past.
Of course, if you haven't used any of these things and you haven't had any problems, then keep doing what your doing. I do think, however, that it is essential for people who use their diapers as nature intended, #1 and #2, and for those who view diapers as paramount to regression and a baby mindset to use baby powder and diaper cream regularly... it is for me anyhow.
So, to answer your question, I use that stuff for the occasional practical purpose i.e. less irritation and for regressive purposes, after all, almost every baby is powdered up before being taped into a diaper and sent on their way