I'm sure you know what "he has been stripped of his official roles" means. But that phrase isn't specific about *when* he was stripped of his official roles". By adding "since", you are adding a specific time element to the sentence. Your example is shorthand for... Formerly the Communist Party leader in Chongqing, he has since (the time that he was Communist Party leader) been stripped of official roles in a scandal that has seen his wife and police chief jailed. But that would be redundant. So simply by adding "since", you are now referring back to a time mentioned earlier in the sentence or in a previous sentence. I was married, but I have since gotten a divorce. I was skinny when I was young, but I have since gained a lot of weight.