Literally for zero benefit. Just shows Ukrainian (as well as any other country's) diplomats that he is willing to dump their back channel into the public sphere on a whim. There is a reason this "strategy" has never been used in the history of statecraft.
I'm not even sure what Trump thinks he's gaining by spreading this? Zelensky is clearly concerned that if Trump takes office again, an unfortunate near 50:50 at this point, that he will be pull all support from Ukraine (maybe even help Russia). He's doing what a man in his position, a good president trying to protect his country under invasion and dependent upon the support of allies, would do. He's going to endear himself to the egotist, puff him up and hope that if he wins the presidency, Trump either sees reason or at least enjoys Zelensky sucking up to him enough to not fuck this up for Ukraine.