At the risk of being obvious, Trump's letter clearly delineates his reasons.
Arguably, Trump has screwed up worse than the WHO did. At the time when China was in full lockdown and European countries were declaring local quarantines, Trump was tweeting about how it all was going to just disappear.
That said, WHO has been of extremely limited value in response to this particular public health crisis. Certainly not harmful, but nothing would have been different if WHO had not existed. The quarantine and distancing measures were implemented on behalf of individual nations, not the WHO. Even the most favorable sources acknowledge WHO's mistakes.
Both parties are at blame here. Both Trump and the WHO grossly underestimated the threat, leading to significant loss of life and greater economic losses than would've been incurred by an early hard quarantine response.
It's not exactly the same, though. The president's job is to judge the priorities, including choosing between lives and money. Kafkaesque as the 2016 election was, the US did choose what appeared to be the lesser of two evils, and national presidents do get that priorities excuse. The WHO, on the other hand, has a singular mission, in which it has failed, allowing Trump to direct the whole of the blame at it.
What the US gains is a way to make a statement. The money in question is fairly inconsequential for the US budget, but it's significant for the WHO. Exercising the ability and the will to control that funding allows Trump to nudge the WHO's priorities in the direction he desires.