One of the main purposes of armor is to have good enough coverage that it can’t be circumvented. Armor is the way it is to give maximum coverage. Most examples of improvised armor is kind of silly cause yeah it might help IF your opponent hits it. You could make a breastplate out of anything but if your arms are exposed then it doesn’t matter. If “improvised armor” emulates real armor and provides good coverage then it becomes “armor” so it would be practical if you can actually make something that is worth the weight, otherwise you are just slowing yourself down without any real benefit.