The heartbeat protocol requires the other end to reply with the same data that was sent to it, to know that the other end is alive. Wouldn't sending a certain fixed message be simpler? Is it to prevent some kind of attack?
At least the size of the packet seems to be relevant, because according to RFC6520, 5.1 the heartbeat message will be used with DTLS (e.g. TLS over UDP) for PMTU discovery - in which cases it needs messages of different sizes. Apart from that it might be simply modelled after ICMP ping, where you can also specify the payload content for no reason.