Narrative text generally utilizes a more creative authorial voice when compared to technical prose. Often in storytelling, the narrative portion of the text will either be told from the first-person perspective of one of the characters, or from an authorial third-person perspective that nevertheless reflects characters' thoughts and actions. An advantage of this is that you can write a more creative, involving narrative for your readers. A disadvantage is that readers are less likely to fully glean underlying implications from a narrative when compared to technical prose, which is more straightforward and transparent.
Use narrative text to convey subjective information to your readers. This is very advantageous in storytelling, as the narrative style itself is a large part of the reader's experience with the story. By narrating from the perspective of a character in the story, or from a limited authorial perspective, you convey subjective or incomplete information to your reader. This can be used to build suspense, mystery and reader involvement. However, subjective information is disadvantageous when you want to convey something to your reader in as simple and complete a form as possible. For such a task, expository writing is better suited.
Related to the creative aspect of narrative text is its power of description. When writing in a narrative style, you are able to exercise your creativity when describing characters, scenes or events. Like the cinematography of a film, creative description sets the mood of a narrative while also conveying underlying implications, often in metaphor. For readers simply looking for information, creative description is disadvantageous, as it will likely seem cumbersome. However, for readers engaged with the text, your narrative descriptions will become a large part of the text experience.
Most commonly, narrative text is used to narrate the bulk of a story, as opposed to the sections of character dialogue. To seem realistic, dialogue generally has to be short-winded and relatively simple in style. Narrative text, on the other hand, can be written in a denser style in which a lot of information is packed. This allows you to narrate the story while conveying any number of other points to your reader, such as philosophical implications. Dialogue can also accomplish this, however the points you make are likely to be vaguer and/or shrouded due to the relative simplicity of dialogue text.