If you're using a real chamois (hard to find nowadays), you need it to soften the chamois after washing. When it dries, it's like cardboard and needs to be seriously kneaded to soften it, then creamed so it doesn't chafe.
If you're using a synthetic chamois (pretty standard nowadays), you need it if you'll be riding in the rain and you don't have a rear fender. A wet synthetic chamois can get EXTREMELY irritating unless it's lubricated.
When I rode PBP, I kept a small tube of chamois creme in my seat bag. Since I was riding a fixed gear, there would be constant friction unless I was out of the saddle, so I had another reason to carry it. It did come in handy after the second day (of three).
For rides that I plan to finish in one day where it's not raining (like double centuries in California), I never bother with it.
Luis