The washer keeps the valve from being pushed down into the wheel while filling, especially when flat and steady in the hole. Another theory is to help so the tube doesn't slip/turn inside the tire when flat possibly...who knows.
Sure, you can remove it...not a huge deal. You don't use nuts on a tubular wheel, because there is no threading and you often run extenders...so it is certainly possible as others said.
You definitely don't want to turn the nut tight either and pull the tube/valve up through the hole, it will cut/ruin the tube likely.