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The Root Mask gives you Poison resistance. I don't have exact numbers but it is slightly weaker protection than the mead provides. However, it never wears off, and I found it is more than enough protection for any poison besides Bonemass. Makes Swamp exploration much less annoying.
The Root Harnesk (body piece) gives you Pierce resistance. This is most notable against Deathsquitos, which become MUCH less threatening. It also helps against archers, particularly the Draugr archers who can hit pretty hard at 1+ stars. Reduces the amount of dodging you have to do against either of these threats.
The Root Leggings do not have a special buff, but they are required for the set bonus, which is +15 bow skill. When I got this armor I was around 20 bow, so this was nearly a 100% buff for my character. The difference in fire rate is definitely noticeable.
For fashion, pair with the Lox Cape and you've got some awesome Wood Elf vibes going on. And armor suitable for exploring almost all of the world map!
Source: https://www.reddit.com/r/valheim/comments/r6563f/root_armor_is_a_must_have_for_swamp_and_plains/