First of all as I understand this rule only apply on a fast bowling and not on the spin bowling. The batsman already wear helmet when a fast bowler is bowling so there is nothing change in this part however the wicket keeper usually stand very far away and there was no need for him to wear the helmet at that time.
It will be interesting to know how the Wicket keepers around the world will react to this new ICC rule which will just create more irritation to them rather than any safety 🛟 🦺
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This is from ICC's briefing
-when batters are facing fast bowlers
-when wicketkeepers are standing up to the stumps
-when fielders are close to the batter in front of the wicket.
It's not just limited to Keeper or batter, as you rightly pointed out that every batsman and Wicket keeper already follow this because of safety. It's just ICC included this in the rule book and made it mandatory.
Also, this rule mainly focuses on fielders (when spinners are bowling), if they are close to batters like short-leg, silly point, silly mid off -on. These are the most dangerous areas to field, there is a reason you only find rookie members of the team standing there with few exceptions.