Can anyone tell me the difference in brewing wine with demijohns compared the fermenting buckets?
In a word, air.
Air, or more particularly oxygen, is bad for wine.
While you've got an active fermentation going your wine is protected by the head of carbon dioxide which is being produced, because CO2
is heavier than air, so a covered bucket is fine as long as you don't keep taking the lid off.
In the later stages, and for maturation, your wine should be in a sealed fermenter under airlock, topped right up to the neck so the airspace is kept to an absolute minimum.