Find out how much a value has changed? [ :Changed() ]

You never update the oldValue which is probably 0 so it will not change the new value.

Secondly you have cariables for the IntValue so use them.

Lastly oldValue should also be a local variable.

Example

-- you should use get service where possible 
-- incase the name if the service is changed
game:GetService("Players").PlayerAdded:Connect(function(plr)
	
	 local leaderstats = Instance.new("Folder")
	 leaderstats.Name = "leaderstats"

	 local points = Instance.new("IntValue")
	 points.Name = "Points"
	 points.Parent = leaderstats

	 local oldValue = points.Value -- use the variable
	 points.Changed:Connect(function(newVal)
		 print(newValue - oldValue)
		 oldValue = newValue -- update the old value variable
	 end)
	
	-- parent to the game last
	leaderstats.Parent = plr
end)

I hope this helps.