Rudy, at the time, was defending PDJT against the Mueller investigation. Mueller prosecuted Manafort for his dealings in Ukraine based on Ukraine sourced payments and tax fraud.
There is no doubt Trump wanted to know WTF was going on in Ukraine related to 2016. Rudy discovered the Biden issue and told Trump the DNC was behind digging up dirt on Manafort.
It’s interesting that Mueller was disinterested in the DNC role in Ukraine related to 2016.
... The Durham investigation also has become politically important to the White House’s defense of its actions in the Ukraine affair.
Acting chief of staff Mick Mulvaney and other allies of Trump have said it was wholly appropriate for Trump to ask Ukraine’s president and other heads of state for help with U.S. investigations that could uncover wrongdoing.
Mulvaney cited Durham’s ongoing work and the need to answer what Mulvaney called lingering questions as one explanation for why Trump had asked the Ukrainians for investigations.
Trump or Barr also have asked the Italian, British and Australian governments to cooperate.