If you have a network and some devices with valid ips configured by either ipv4 local link, apipa, dnssd or zeroconf if you do reboot one of them.
Does the rebooted device get the same ip as before if nothing changes in the network?
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.