A non math way of explaining the need to compare the USA economy with other countries economies would be this...
If you want to isolate the impact political leadership you need to understand all factors that effect the economy. That way you can evaluate which factors are effecting things. This is why you look at what's happening in other areas of the world. There is a common issue that the entire world is experience due to COVID. Those factors are there for everybody so you can't point the finger of blame at one party in the USA.
What you can do is see how we have responded to it and how we have performed in comparison to the rest of the world.
Are we under, average, or over performing the rest of the world... Thats how you judge the impact of how we are handling the situation on a policy level.