Like I said earlier, it's to protect the player.
The reasoning would be more for younger players who have been signed on overs, but would still apply across the board. If, say, an unscrupulous club like, say, the Roosters realised that they'd paid overs for a young player that didn't live up to expectations. If they wanted Nikorima, sorry, "nameless player" gone, they could either play him in reserve grade, or even just sit out the year, knowing that it would damage that player's chances of getting a new deal anywhere else in the future. They would do that knowing that the player would much rather take a lesser deal elsewhere in order to keep playing, and happily negotiate a "mutual release", that way the Roosters would not have to pay a cent towards the shortfall. To prevent that from happening, and to protect players from dodgy club dealings, once a contract is lodged with the NRL, that player is guaranteed that money while ever they're still in the NRL, and the club that registers the contract is effectively a guarantor of that value.
I could ask the NRL what the rules are, but I doubt they'll even know. And unfortunately the Roosters seem to have a fridge full of their player's failed drugs tests that they can raid whenever they need a loophole.