It's more of an accidental side effect than an intentional feature. The actual reason the customer gets 2^64 addresses is to make sure they have enough addresses and don't need NAT. And SLAAC (also an accidental feature) ossified it at 2^64 - a good ossification, for once. And then, if you have so many addresses, may as well rotate through them so it's hard for anyone to observe how many separate devices from your network are accessing their server.