The technique was only really useful before the entire display could be addressed directly with high speed CPUs attached to large frame buffers, that is, in the days of 8-bit systems. In this case the CPU was not terribly fast, and that 10 uS might be enough to run perhaps 40 to 60 instructions. That was enough to change a few registers in the display hardware however, which is why this technique was useful.
For instance, both the Bally Astrocade and Atari 8-bit family (originally intended to be a console) included HBI support. Both could display only four colors per pixel, but which four could be selected from a palette of 256, selected by storing the color number in one of four registers. By changing the values of the palette registers during the HBI, the system could select a new set of four colors on every line, leading to a number of "rainbow" displays with all 256 colors on screen (four per line).
See also: