At this point, Republican voters aren't quite sure whether DeSantis or Haley is better, but they do know that Trump is the worst one of them all and must be defeated. As DeSantis' campaign has been struggling since its launch, his hope has been to retain the support of the donor class — with the Koch endorsement, however, Haley will not only gain the incredible funding and grassroots activism provided by AFP but the donor class may change their mind on who they'd prefer to back.
It's unclear why the Koch network hates Trump so much, given that under his presidency, many of its preferred policies were enacted. While this could be seen as a boost to the Haley campaign, Trump is way ahead of any other candidate with a national average of 61.6% in the polls, compared to Haley's 9.8%. AFP seems to be giving it its best shot, but Trump's popularity and AFP's nonexistent policy qualms with the former president make this a curious decision at best.