Subcultures are groups within a larger culture which share some binding element of shared experience. This can be everything from people who are all fans of the same TV show, band, or book to people who all attend the same church, work in the same field, or belong to the same club. These are simplistic examples, of course, but they'll do to start. Groups like these build a culture around themselves. Broadly speaking, fans speak in catchphrases, church attendees reference shared experiences, colleges speak the language of the trade and club members have secret handshakes. All of these things constitute a way of communicating ones belonging to the group, but they're also more than that. They're a way of recognizing like-minded souls.
Humans find things they have in common and use these things to relate to the larger world. We often categorize ourselves by the groups that we belong to, beginning with the most intimate (families) and moving to the more general in degrees. Few people identify themselves with the book club they belong to, but many identify themselves by their spirituality, occupation, or hobbies.
Subcultures not only show a person's perspective, and help them communicate it to others, but even shape it. Over time, we begin to think like those with whom we associate, our perspectives change based on the input and perspectives of those around us (and the degree to which we are comfortable with those perspectives). So, when writing fantasy and science fiction, I think it's helpful to think of the subcultures to which our characters belong.
For an example, in my novel Born of This Soil, the MC is Beshauna. Beshauna is from a culture with a big, deep history, but her nation is occupied by another culture and largely ruled by it. This makes her native culture a subculture of the larger, (somewhat) blended culture. Beshauna heavily identifies with her native culture, with its religion and its values, and throughout the book her perspective is influenced by that. She sees everything through that filter, comparing each new experience, person, and place to what she knows, and her decisions are often based on what brings her closer to that culture (and her conception of it) and what pushes her farther from it.
She is also a member of smuggling ring which sneaks magically-talented people out of the country and to safe havens in other countries. Despite the moral nature of her operation, this puts her in the criminal subculture. She is more at home with criminals than with authority figures, she is more at ease among them, and she is more likely to understand them--especially if they are also part of her native culture--than she is likely to sympathize with others.
These perspectives are a huge part of who she is and writing her means knowing these subcultures inside and out. Because she is so closely tied to them, and the perspectives that they promote, share and engender, when I write Beshauna, I always have to keep in mind where she comes from. I have to know their rituals, the language they use (both with one another and with "outsiders" to the subcultures). To effectively portray her interaction with these subcultures, I have to know who they see the world, and how Beshauna's perspective is the same and different.
So, to write Beshauna, I have to write the subcultures. To characterize her, I also have to characterize them, and the two feed from one another. The more the reader understands Beshauna, the more they understand her world and vice versa.
While not all characters are as tightly bound to their subcultures as Beshauna, they still exist within them and come from them. If your character is a knight, what does that mean to them? What does that mean when they meet other knights? How do they interact with one another, and how does that differ from their interaction with other groups, or outsiders to the knighthood?
Subculture presents thousands of options for characterization, from language (including inclusive language, exclusive language, in-jokes, and the language of shared experience), to food, clothing, humor, habits and religion. Subcultures are the meeting of character and culture and provide an excellent opportunity to deepen both your world and your characters, at the same time. They're twice the bang for your word count buck.
And who doesn't love efficient worldbuilding, am I right? :-D