I cannot help feeling terribly excited as the time draws near for the visit of Dr. Van Helsing, for somehow I expect that it will throw some light upon Jonathan's sad experience; and as he attended poor dear Lucy in her last illness, he can tell me all about her. That is the reason of his coming; it is concerning Lucy and her sleep-walking, and not about Jonathan. Then I shall never know the real truth now! How silly I am. That awful journal gets hold of my imagination and tinges everything with something of its own colour. Of course it is about Lucy.
I don’t want to lose sight of this moment, even though it is resolved in the same day. Mina is so good at recognizing the relevant details.
She hasn't spoken to Van Helsing yet. She doesn't know about Lucy's blood loss, outside noticing she seemed somewhat anemic in Whitby. But already she's connecting the dots. Jonathan's journal mentioned Whitby circled on a map, he saw the Count in London. The red eyes are also something he mentions, though not as dramatically as Lucy. She can piece together the lizard fashion wall-crawl and Lucy by an open window even if she doesn't know about the shape-shifting. Mina may not be listing these things off out loud, or even consciously, but she knows. She has recognized the significance of all of this, especially when pairing with a doctor who no doubt wants to talk to her about Whitby.
And she tries to deny it to herself, calls herself silly. She has no proof of any of this yet. She must be overreaching. (She's not and she knows it. She typed up both journals since yesterday.) It echoes what Jonathan has been trying to do ever since his escape, to deny and rationalize everything away as imagination or madness.
But that hasn't worked for him and Mina knows it won't work for her. Van Helsing tells her nothing really. He mostly just thanks her for the info, but she breaks down and gives him Jonathan's journal because she's figured it out on her own and only wants confirmation and support from someone else. She needs validation, but she already knows. Because she is clever and she cares, and she is able to remember relevant details and piece them together.
That is, in the end, what she does with this whole narrative, isn't it? And if it weren't for that ability Dracula would never be defeated. Not getting too far into spoilers, but the conceit of the entire novel relies upon these relevant journal entries and letters and newspaper articles being noticed and collected and we all know who deserves the most thanks for that.
Not to mention the significance of Mina and Jonathan being the first people to allow someone else to read their documents! Arthur (and somewhat Seward) gather people, but no one has shared much information yet. Van Helsing read Lucy's memorandum (an open letter meant for after her death) secretly, and then only got secondhand permission to go through her other things.
The only people so far who have actually physically handed over their records and told someone else they can read them are the Harkers. First Jonathan, giving his journal to Mina - but he was still trying to deny what happened to him, and asked her not to read it unless she needed to do so. Mina has been the only one so far to recognize the need and then immediately take steps to share information with others. Jonathan and Mina are the only people who have written in code (specifically shown to hide information, essentially from Dracula, but also briefly in a humorous way from Van Helsing). And yet they're the only ones to share that same information, and Mina is the one to outright transcribe and collect it for that specific purpose.
If anyone was doubting their importance to the story, that right there shows how essential they both are. Because Dracula is a novel about trust and love and document review vanquishing Count Dracula. And they, but especially Mina, are the ones who first and foremost demonstrate all those qualities.