At the risk of being obvious, Trump's letter clearly delineates his reasons.
Both Trump and WHO have made mistakes in regard to this crisis. At the time when China was in full lockdown and European countries were declaring local quarantines, Trump expected it all to just disappear.
That said, WHO has been of limited value in response to this particular public health crisis. The quarantine and distancing measures were implemented on behalf of individual nations. Even mostly-favorable sources acknowledge WHO's mistakes. Both Trump and the WHO underestimated the threat.
There is a difference, though: the president's job is to judge the priorities, including choosing between lives and money. Kafkaesque as the 2016 election was, the USAmerican voters did choose what appeared to be the lesser of two evils, and nationalat least in their opinion. National presidents do get to choose what to prioritize (inprioritize; in this case, Trump prioritized the economy over safety), rightly or not. The WHO, on the other hand, has a singular mission, protecting the public health, in which it has failed.
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 affect the WHO's priorities.