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Right saddle/setup/bib shorts.
I find that I get numbness well before any friction related issues, but do use for spot/saddle sore prevention.
Acliff-
I used it when I got into Audax as I saw many others talking about it. Then I forgot one day and did a 200k without it, and nothing bad happened, never used it since.
For rides like LEL and PBP I just switched shorts each day and made sure that whole area was clean (baby-wipes if I couldn't get a proper shower). The last day of LEL I wore two pairs of shorts (since it was mostly the flatlands and I was on fixed).
(I'm also odd in that I don't go commando in cycling shorts. Again, I've never had any specific gooch problems on multi-day rides, but then I've only ever gone up to 1400km and never anything like TABR or TCR.)
(But I do understand people do have problems and so it may be necessary. But the whole 'you must otherwise' I don't believe in since I'm a single datapoint that proves otherwise.)
Greenbank
They mask problems that can be solved by other means (finding the right saddle/setup).
But if you've never found the right saddle/setup then chamois creams can get you through what you may not be able to suffer otherwise.
(And then some people just can't do rides longer than 1h without chamois cream regardless of what they do.)