What happens when you show up and don't have an appropriate size for the gentleman? Heck, I'm not even an escort and I keep three sizes of condoms in a little pouch in my purse, just in case.
Many guys are married and I'd think it was a pain for them to have to stop at the gas station and buy a three pack of condoms on the way to an appt.
Additionally, I'd want to know the condoms were the appropriate type and in good condition.
On a related note...if a client pays you then asks if you have condoms. Your answer should be "for what?" This is a common way LE will prove that you have offered sex for money. Same thing with, "is it extra for Greek?" Which may at additional penalties for sodomy/crime against nature.
Just a little public service announcement for the day.