I think /64 was the guideline for the smallest divisible network an ISP should hand out. It basically allows for a household or organization to grow indefinitely while assigning anything that can hook up to the network a real IP address, with little restrictions on topology and/or address allocation layout. No need to reuse the addresses either, just give the new device a new one.
EDIT: I forgot to add that it also allows network devices to use their 64-bit link layer (ethernet/MAC) address as a host identifier.
Actually, /64 is the smallest indivisible network. You can divide that further, but then you lose this network/host identifier split and IPv6 features that require that (stateless autoconfiguration, privacy extensions...). So in case of normal network, ISP handing out /64 restricts topology of you network in that you can have only one network (with normal end-user devices).
Using smaller networks than /64 is certainly possible, but it's not something you want to do on end-user network and it's best left only for things like point-to-point connections and intra-datacenter networks.
EDIT: I forgot to add that it also allows network devices to use their 64-bit link layer (ethernet/MAC) address as a host identifier.