For prepare and measure protocols, the preparation stage requires a perfect random seed to not know in advance the send state, which is a loophole allowing an adversary party to get access to the full extent of the shared key.
To avoid such scenario, the only way to get a perfect random number is through quantum, with a quantum random number generator (QRNG).
QKD performed with entanglement distribution presents naturally high entropy, that can be translated to randomness coming from a quantum process.
Entanglement based QKD allows for quantum key generation with only an entanglement quantum source, and not a combination of a single photon quantum source with a required QRNG.