dyna-ti makes a good point (regarding wet down not the Mt Everst one…) 🙄
Down is useless when wet, synthetic retains some insulating properties but water will conduct heat away 20 times faster than air regardless. I’ve sat awake all night wet and cold in winter bivvies using both – my fault as I’d made mistakes when packing.
Regardless of the type (synth opr down) its best to invest a couple of quid in a dry-bag to keep your sleeping bag in. If you can stretch, a water resistant outer will add protection when you’re using it.
Folks have also mentioned silk liners, again for a couple of pounds these up the comfort and heat rating quite a bit.
Regarding why down – good quality down is lighter, packs smaller and if you look after it, keeps you warmer making for a better trip all round.