Should I use SetAsync or UpdateAsync?

The developer wiki says that SetAsync shouldn't be used when updating a data store, and that UpdateAsync should be used instead. But what if I wanted to update it to new data?

Let's say I wanted to store a player's kills:

-- Correct me if there's something wrong with my code

local DataStoreService = game:GetService("DataStoreService")
local killsStore = DataStoreService:GetDataStore("PlayerKills")
local playerKills = nil

game.Players.PlayerAdded:Connect(function(player)
	killsStore:SetAsync(player.UserId, 0) -- Pretend it checks if the key already exists before this
	playerKills = killsStore:GetAsync(player.UserId)
	player.Kills.Value = playerKills -- Kills is an IntValue here
end

And then I wanted to update the data store. I could do this:

local function getNewKills()
	return playerKills
end

game.Players.PlayerRemoving:Connect(function(player)
	playerKills = player.Kills.Value
	killsStore:UpdateAsync(player.UserId, getNewKills)
end)

But if I'm just going to overwrite the previous data, why not use SetAsync instead?

game.Players.PlayerRemoving:Connect(function(player)
	playerKills = player.Kills.Value
	killsStore:SetAsync("Player_1234", playerKills)
end)