Krogstand opened the doors to the real world to Nora. All this time she had been a "doll to be played with" and her naivete about the world was such that when she realized that this man was blackmailing her and threatening to break her fantasies forever, she did a huge jump into the reality of how obscure some people can be, and that nothing happens without consequence.
Nora understood very little about consequences, and about evil before she made the "business transaction" with Krogstad. Even as he continued to threaten her, she was in denial. In all, he was the first taste of reality she had in her adult life.
Mrs. Linde was the second instance of reality: This time, it was the reality of a woman who is not married to money, and who has to earn her living. Nora herself says how sorry she felt for Ms. Linde and clearly showed how shallow and superficial her own life had been as she compared herself with Ms. Linde, who has experienced deception, death, loneliness, and sadness. Mrs. Linde is a dramatically contrasting character, and shows how clueless Nora was of how other people lived their lives. She lived, literally in a bubble until then.
Dr. Rank showed Nora the reality of death, and limits. At the same time, he showed her love. He declared to be in love with her, making her perhaps question her role in her marriage since her husband never really saw her in the light of a lover like Dr. Rank. When Rank received his prognosis, she was devastated and realized that things, every single thing, comes to an end. Just like her fantasy was about to come to an end as well. It is with his departure, coincidentally, that most of the story begins to wrap up and the end is imminent.