Compression is used two ways here: as treatment in its own right for venous symptoms, and alongside a procedure. After sclerotherapy you will wear stockings for fourteen days, and that is part of what makes the treatment work.
It may be recommended for varicose veins, venous reflux or chronic venous insufficiency, swollen feet, ankles or legs, aching, tired or heavy legs, venous skin changes, venous leg ulcers or a history of ulceration, swelling after a deep vein thrombosis, symptoms from prolonged standing or sitting, and during pregnancy.
You can have symptoms of venous disease without visible varicose veins, which is why the ultrasound matters even when your legs look unremarkable.