When you wear any of the pieces of equipment that contain the "Witcher" symbol, you get two passive bonuses. You don't need to spend any of your stamina to get them. The bonus is applied even if you're not in combat.

When you're not wearing any of the pieces of equipment that contain the "Witcher" symbol, you're still able to use the bonuses. It's just that you don't need to spend any of your stamina.

This is exactly the same as the "Werewolf Decoction". It gives you a very small advantage when you're in combat. It is done for two reasons. One if you want to use it, you can and will get better bonuses. Two, it is always active, being immune to too much isn't good for balance.

Geralt is immune to the effects of Stagger. This is done for a couple of reasons. One, if you want to use the Decoction, you can and will get better bonuses.