Say Canadian Whites and American Whites both have mean lifespan of 81, and Canadian Blacks and American Blacks both have a mean lifespan of 71. Using that data and the fact that the US has 3X the proportion of blacks, you could calculate how much of Canada's higher life expectancy is due to demographics. It could explain the entire difference!
Alternatively, if Canadian Whites and Blacks live proportionally longer than American Whites and Blacks respectively, you would know that demographics could not explain the difference.
I mean, okay, but what if black people live 10 years less than white people in such statistics?
That's still an abysmal thing, as the US would have something like 30% of their population getting clearly worse health and not caring or doing anything about it?
If the difference is down to demographics, surely that's really bad?
Maybe so, but touting Canada's "superiority" over the U.S. would lead to the "obvious solution" of getting rid of black people. If the difference is down to demographics, you want to know that, rather than look at people in country-sized blocs.
Alternatively, if Canadian Whites and Blacks live proportionally longer than American Whites and Blacks respectively, you would know that demographics could not explain the difference.
Hence the need for the disaggregated data.