We are a commune of inquiring, skeptical, politically centrist, capitalist, anglophile, traditionalist New England Yankee humans, humanoids, and animals with many interests beyond and above politics. Each of us has had a high-school education (or GED), but all had ADD so didn't pay attention very well, especially the dogs. Each one of us does "try my best to be just like I am," and none of us enjoys working for others, including for Maggie, from whom we receive neither a nickel nor a dime. Freedom from nags, cranks, government, do-gooders, control-freaks and idiots is all that we ask for.
" How would it be possible for a Ukraine investigation into Biden to be good for Trump politically unless it was also the kind of thing that voters in the United States would care about?"
"There were two potential outcomes of the requested Ukraine investigation on Biden. Either they would find nothing, or they would find behavior that voters in the United States need to know. Which of those two outcomes is the impeachable one?"
It is obvious unless you are so far left you can't see truth from your front door, that Biden, Obama and Hillary used their positions of power to extort Ukraine's corrupt government to give them kickbacks. Biden's method of choice was to get his son appointed to positions that appeared legit but clearly were not. This mostly happened inside the Ukraine. Virtually impossible for the U.S. to investigate but quite possible for Ukraine to investigate. So the best possible way to do this would be to ask the government of Ukraine to investigate this corruption. Even if there was arm twisting (quid pro quo) so what? No one is above the law and Biden broke the law and he needs investigation as do Obama and Hillary.
I draw a sharp distinction between using the power of the U.S. Presidency to induce either U.S. or foreign officials to investigate something an U.S. office-holder did, vs. something a mere aspirant to office did. Either may provide fodder for a campaign, but one is using the power of one's office to attack a merely political foe, while the other is legitimate law enforcement. If it's law enforcement, I don't see why the presence or absence of a quid pro quo matters in the least. If it's simultaneously law enforcement and something that will be useful in a re-election campaign, tough. You could say that about anything a President does while in office to help his country: "He didn't do it to help the county, he just wanted to be re-elected!"