The Mueller report by installments

Second: it's not primarily ABOUT Cohen. See, this is bouncing the rubble. It adds one more felony to those Cohen has already taken, with no real marginal effect. It will have an extremely negligible effect, IF ANY, on his sentence. So why bother?

Well, normally a federal prosecutor WOULDN'T bother with a plea that has no impact. But here, the impact is telling the story of the investigation as a whole, and promoting the case that the entire Trump Organization was lying about its degree of involvement with Russia.

Though Trump isn't named explicitly, he and his organization figure prominently and obviously in the charging document and plea. That's not something federal prosecutors do lightly. This strongly implies further action against someone.

Now, we don't have the written factual basis yet (the statement of supporting facts that Cohen agrees to). But reports of Cohen's plea suggest he admitted to these lies to help Trump and Trump's narrative, but notably NOT claiming that Trump knew he'd lie to Congress.

But the conclusion that the President of the United States knew that his personal counsel was repeatedly lying to the Congress of the United States about the President's business is inescapable under these circumstances.

It was awfully short-sighted of His Lardship to turn Michael Cohen out in the cold.

...but hey, do what you want...you will anyway.