I think a more fruitful question is: why did someone leak this letter now that was written four months ago partly by a team that no longer exists because the lead resigned in March?

As this June 4 piece @ Newsmax points out: 

Even Rudy Giuliani on Sunday backed away from the Sekulow-Dowd assertion.

"Pardoning himself would be unthinkable and probably lead to immediate impeachment," Giuliani said. ''And he has no need to do it, he's done nothing wrong."

And I've seen elsewhere where others agree, like Chris Christie having said something like "if he did that, he will be impeached."

I went back to the original New York Times piece introducing the letter and the Times merely says they "obtained" the letter without a hint from who or why except this, my underlining

Mr. Trump complained on Twitter on Saturday before this article was published that the disclosure of the letter was a damaging leak to the news media and asked whether the “expensive Witch Hunt Hoax” would ever end.

Donald J. Trump✔@realDonaldTrump

There was No Collusion with Russia (except by the Democrats). When will this very expensive Witch Hunt Hoax ever end? So bad for our Country. Is the Special Counsel/Justice Department leaking my lawyers letters to the Fake News Media? Should be looking at Dems corruption instead?

1:43 PM - Jun 2, 2018

Though it should be noted that this reaction could have come from the Times informing him they were going to publish it.

I have seen several pundits I respect suggest that raising all this talk about pardoning is a distraction.

As to it being a legal argument back in January by a lawyer that resigned in mid March, I just see that as one thing that a lawyer defending this president might correctly try to see what the response might be. I also see that there was a hullabaloo about Dowd fiddling with that idea already in taking responsibility for a tweet by Trump in early December, see the update atop this Dec. 2 story at Washington Post