Filling the disk with random data prior to encrypting it will supposedly make it harder for the attacker to perform any cryptanalysis. Most sources seem to state this is because it will be harder for the attacker to determine what data is actually encrypted (and which is just random garbage).
However, is this strictly necessary? It can take a prohibitively long time to fill the entire disk with random data, for large disks. If the data could be attacked and decrypted with any form of attack, then how much is this extra hurdle really worth? What is the real concern and attack scenario where this sort of prevention technique is actually any useful?
Has any encrypted data ever been decrypted because the owner failed to fill the disk with random data prior to encrypting it? Or is this practice just an overly paranoid extra measure that in reality provides no real additional security?
If needed to provide any specific answer, then assume the system is GNU/Linux with LUKS, AES 256 bit, having encrypted data on a normal HDD. Two partitions: One /boot partition with no encryption only used for booting and one root partition with said encryption.
Attack scenario: The attacker obtains the computer with the power turned off. Assuming no cold boot attack or evil maid attack is possible.