Why was the Christmas present for Eurus so important?

Question

In BBC Sherlock, Season 4, Episode 3 The Final Problem there is a scene, in which

Jim Moriarty sees Eurus in her prison. It is some kind of a Christmas present for Eurus from Mycroft.

Later in the show all protagonists behave as if Mycroft's decision was a very important mistake that ruined everything.

I don't get it. As far as I understand it, this meeting changed nothing. They met... and what? Yes, they created a few funny videos together, but that doesn't seem very important for the story. Why was it so important? Did the meeting influence their characters in any important way? Would they behave differently if they didn't meet?

Answer 1

Well, God knows what happened to the script writers this season(if there were any). The episode's sole aim was to fool the viewers, I presume. Example, the delayed appearance of "5 years ago subtext", when the helicopter carrying Moriarty lands at Sherrinford.

Coming back to your question. From the second season it is quite clear that Moriarty is a twisted, rich and influential man. Icing on the cake, he is also interested in destroying Sherlock, just like his sister.

PROSECUTING BARRISTER: How would you describe this man – his character?

SHERLOCK: First mistake. (He raises his eyes and locks his gaze onto Jim.) James Moriarty isn’t a man at all – he’s a spider; a spider at the centre of a web – a criminal web with a thousand threads and he knows precisely how each and every single one of them dances. (Jim almost imperceptibly nods his head as if approving of the description.

Season 2, episode 3

So Eurus asked for a meeting with him. The tapes recorded by him sent the entire govt baffling as to what had happened. Even, Sherlock took an overdose to prove that Moriarty is not alive - The Abominable Bride episode.

In my opinion, apart from recording the tapes, Moriarty provided Eurus resources(monetary as well as other) that helped her gain control inside and outside the prison. Moreover, I believe, Moriarty also provided her with valuable information on Sherlock, the context she needed.

Since Eurus could actually control the entire prison, I don't think this was the only meeting she had with Moriarty. The 5 minute meeting was basically a foundation stone that spelled the doom for Sherlock and all the people close to him.

Answer 2

Since Moriarty's involvement in the events of Series 4 are a plot device, the details are unclear. We are led, I think, to conclude the following:

Moriarty saw Eurus as an ally in a long-range plan to destroy Sherlock if Moriarty himself were defeated or killed. Sherlock is hinting at this in The Six Thatchers (transcript source: ForeverDreaming.org):

SHERLOCK: He's planned something, something long-term. Something that would take effect if he never made it off that rooftop alive. Posthumous revenge. No, better than that - posthumous game.

The use of the Moriarty videos allows Eurus to command Sherlock's attention with the 'Miss Me?' message throughout the series (actually, Mary Watson ironically piggybacks her own posthumous messages on this). Sherlock does not know who Eurus is, so she pulls the strings from behind Moriarty's disembodied identity. Eurus' purposes all centre around Sherlock so she needs the Moriarty persona to control his actions and reactions until she has him at Sherrinford (and even then uses the videos to taunt Sherlock).

Eurus, prior to absconding from Sherrinford, has had no direct contact with Sherlock since childhood and so will have used Moriarty's knowledge of him to help construct her plan; in this Moriarty has acted as consulting criminal (transcript source):

EURUS: Jim Moriarty thought you'd make this choice. He was so excited.

MORIARTY (recorded): And here we are, the end of the line. Holmes killing Holmes. This is where I get off.

SHERLOCK: Five minutes. It took her just five minutes to do all of this to us.

Finally, this question raises the possibility that Moriarty was the agent who obtained the original letter by Faith Smith in The Lying Detective (transcript source):

EURUS: Culverton gave me Faith's original note. A mutual friend put us in touch.