The most important benefit of
well fitting compression stockings is in the prevention of deep vein thromboses (DVTs). DVTs can develop when blood pools in peripheral veins, as during a long flight. Upon resuming activity such as walking the clot may break loose and cause obstruction of an artery which can cause issues including death. In addition the gentle compression can reduce peripheral swelling due to pooling of blood and lymph in the lower extremities. This also can be protective of the veins and their valves in the lower legs by reducing the intravenous pressure which can result in dilitation of the veins and produce incompetence of the valves.
Compression stockings are made to provide varying degrees of compression pressure. As in many things in life "you gets what you pays for" and I'd be concerned about inexpensive compression stockings - mine usually run around $30 - $40 and seem to work well - at least I'm not dead yet!
I realize it's already to make a long story short but I have found what works well for me is to wear the compression stockings under my walk socks in the manner of a liner.
Buen (and safe and healthy) camino.