IIRC, the Impala spindle ("B" Body) was the way to get bigger brakes (and maybe a slight height reduction?) on the Chevelle ("A" Body) front end via the lowest cost method, i.e. junkyard. The popularity carried over from the use of the A Body front clip in a number of oval racing series (which continues to this day, I believe, in some association rules); you know, the low cost, claimer, IMCA, and other track rules before they blew up into allowing non-OEM stuff. That they're in the Speedway catalog shows that they still must be used in some (probably Midwestern) series.
vic