How does wrapping my leg help with venous stasis ulcers?

Lowers vein pressure. Wrapping the leg with a venous stasis ulcer will dramatically decrease te pressure in the veins. This reverses the process which caused the ulcer in the first place. Healing does not occur overnight. It will take weeks to months depending on how large the wound is.
Yes. However the actual wrapping or compression bandage has to be placed by someone that has training in this procedure.
Decrease pressure. Venous stasis ulcers are due to venous hypertension which is usually due to either increased back pressure in the saphenous system or perforator systems or both. This is due to malfunctioning (refluxing) valves in these veins. Compression helps to decrease venous hypertension and increase venous return. A venous reflux ultrasound would identify the problem. See a vein specialist.
Multifactorial. Wounds need to be addressed in several ways. Some factors include: infection control, wound debridement, and compression therapy.
Moves fluid. Venous leg ulcers are related to pooling of blood in legs and fluids in tissues compression literally squeezes it outa there.
Venous disease. Venous disease is the disorder; compression is ultimately the path to control. Due to abnormal blood flow in the return of blood to towards the heart, the pressure inside the most distal leg veins become distended, and fluid eventually leaks through the tissues to the skin and ulcer develop. Compression, the level at which needs to be determined for your safety in the way to control this condition.
Reduces pressure. It also reduces inflammation. As dr. Salartash said, it is important for the wrapping to be applied by someone who is experienced. Just applying a wrap without proper compression isn't helpful.