Used it for years without asking myself why, then weened my self off it and realised it is a waste of money. I have since done rides up to 300km without Chamois creme... no difference.
Some people swear by it, personally I've found a couple of saddles and decent padded bib-tights and shorts that mean I've never needed it. Only cream i use now is sun-cream if i remember before riding.
Get some Udderly smooth moisturising cream. Better value than the chamois cream. I don't use it all the time but it's good on wet rides if there's a chance of chafing.
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Not tried shammy cream though