Aerating and possibly oxidyzing the beer by letting it fall through the oxygen in the bottle. Wands fill from the bottom of the bottle to the top, pushing out the o2 as it fills. That doesn't happen from the spigot directly, it's falling INTO the air in the bottle.
What you want to do is affix a hose to it, and use it sort of like the bottling wand to fill slowly from the bottom upward.