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.

Hence the need for the disaggregated data.