Should wood be treated with varnish?

Bob Strudick, by email Stefan says: You’ve seen a recommendation to treat wood with varnish but believe it will seal it, prevent it from breathing and cause it to decay. It won’t cause decay, but if the wood is damp and there’s fungal growth already present, it’ll seal it in. Wood should only be varnished when it’s completely dry and sound. Otherwise, use a water-based paint.