One common scenario is in school settings. When other students are bullying Harry or trying to harm him, Draco steps in to protect. Another could be during dangerous magical events. For instance, when a dark magic spell is about to hit Harry, Draco uses his own magic to shield Harry.
Often in fanfiction, there are scenarios where Harry is being targeted by Death Eaters or Voldemort's supporters. Draco, having grown up around them, understands their methods and motives better. He may then choose to protect Harry, either because he has changed his views on Harry or because he realizes that Harry's death would mean a greater victory for the dark side. This creates a very tense and interesting dynamic in the fanfiction stories.
In some fanfictions, when Harry is in a dangerous situation related to his fame as the Chosen One, like being hunted by dark wizards outside of Hogwarts. Draco, who has some knowledge of the dark wizarding world through his family, uses that knowledge to keep Harry safe. Also, in situations where Harry's life is at risk during Quidditch matches due to some foul play, Draco might be the one to stop it and protect Harry.