What is the purpose of the military blousing strap?

"Blousing the boots" or "Blousing the trousers" means to make the trousers appear to be tucked into your boots. Instead of actually tucking in the trouser bottoms into the boots you simply take the elastic straps and wrap them around the ankles and tuck the trouser bottoms into the straps. Now it appears that the trousers are tucked into the boots.