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I think the watering idea is to make them slightly less kindling like. Although, I'm not sure how true that is or how much difference it makes to their combustibility.
Yes, some people do put actual burning candles on their trees, which I'd never do. I'm not sure, either, if that would make such a difference to their combustibility, but if you're going to tempt fate, why not try to fight it at the same time? :)
The best Christmas tree holders are this kind, with the ratchet mechanism:
https://www.krinner.us/christmas-tree-stands
(Just the first search result, no idea what other brands are available.)
I don't think there's any point in watering Christmas trees, unless you have a permanent one in the garden.