I have to know: Why did Comey send that letter about Hillary's emails when he was actually

[Samantha]

55. And here is Comey's letter to Congress on October 28th on reopening the Clinton email case

On October 28, 2016, less than two weeks before the presidential election, Comey announced in a letter to Congress that the FBI learned of the existence of emails that appear to be pertinent to the investigation of Secretary Clinton's email server and the FBI will take steps to allow investigators to review these emails "to determine whether they contain classified information as well as to assess their importance to our investigation." Director Comey stated in the letter that he was writing the letter to "supplement his previous testimony" before Congress.

https://ruleofreason.blogspot.com/2016/11/jail-house-rock.html

and from the same link:

Here is the letter:

To all:
This morning I sent a letter to Congress in connection with the Secretary Clinton email investigation. Yesterday, the investigative team briefed me on their recommendation with respect to seeking access to emails that have recently been found in an unrelated case. Because those emails appear to be pertinent to our investigation, I agreed that we should take appropriate steps to obtain and review them.
Of course, we don’t ordinarily tell Congress about ongoing investigations, but here I feel an obligation to do so given that I testified repeatedly in recent months that our investigation was completed. I also think it would be misleading to the American people were we not to supplement the record. At the same time, however, given that we don’t know the significance of this newly discovered collection of emails, I don’t want to create a misleading impression. In trying to strike that balance, in a brief letter and in the middle of an election season, there is significant risk of being misunderstood, but I wanted you to hear directly from me about it.
Jim Comey

I believe the problem of the misunderstanding arose because Comey informed DOJ he was going to send the letter. The DOJ was planning on a response to that information but Comey not knowing the Department might object and was formulating a response sent the letter the next day. He had in fact been asked to keep Congress informed of further developments and he had promised to do so.

I believe what you quoted might have resulted from pressure from the DOJ which was in fact furious about the matter. It was a violation of DOJ policy and as you know a furor erupted from several arenas over the matter.

Sam