Nothing because you decline "the benefit".
Some do this because they don't want to be rewarded out of ideology or because it would be inappropriate. For instance take Ned as example, he was the ceo of steemit. Should he be paid steem to make a message while it's supposed to be his job in the first place? A ceo is supposed to keep us updated on what's happening, he should not be rewarded for that. That's why he declines payouts.
If you don't need the post reward (money), you can decide to decline your post payout. Declining post payouts is mostly done by the steem Inc officials or team just like Ned does.
I don't see any advantage or benefit and disadvantages in declining your post payouts because it is just a matter of choice. I don't decline my post payout because it's my choice.
Thanks for reading and I hope this helps