Normally it should, according to this IETF draft. However, if for some reason any other device has claimed that address in the meantime, the device must choose a different address.
The automatic address generation needs some randomness, to avoid having all devices choosing the same address (or sequence of addresses). Therefore the mechanism needs a random number generator (RNG). The seed is some device-specific data to initialize the random number generation. A MAC address is a good candidate for this, because it's (supposed to be) unique for every network interface.