LTO - blending and refining

[DanilKa + 443]

I have genuine question to those familiar with dark arts of oil refining:

If we mix equal parts of heavy oil (say 20 deg API) with light crude (say 40 deg API, similar to that produced from "shale"), we'll get ~30deg API mixture suitable for refineries. Aren't light fractions will come out of top of the distillation column without increasing yield of diesel/kerosene? What is the advantage of blending - just to permit heavy crude to enter refinery or refiners aren't checking fractional composition of the feed-stock???

https://srsroccoreport.com/the-united-states-a-net-oil-exporter-the-dirty-little-secret/

thanks!