Disclaimer: This is literally a novel. I am so sorry!
After re-reading my answer, I realized that my definition for pre-professional tracks was really vague. Sorry about that!
As a STEM major, I’m most familiar with the pre-med and pre-vet tracks. I will be basing my response on my exposure to people in those tracks.
The importance of labeling yourself as pre-med/pre-vet is that it signals to all students and faculty that you’re going to be in need of specific advice.
Every Trinity College (non-engineering) student is assigned an academic advisor prior to declaring their major, and then receive a major advisor within their department post major declaration sophomore spring. These advisors would need to know that you’re pre-med/vet because there are unspoken standards that these students need to meet.
These standards come from pre-professional schools, and include the they expect you to have taken and what activities demonstrate your interest and skill in the topic.
This is extremely important, for example, if someone is a humanities major who intends to pursue medicine. A philosophy major who is pre-med will not end up taking these required classes by accident.
At Duke, we have a lot of flexibility in how we design our education. We don’t have common core classes. Beyond your major requirements and the loose framework that forces students out of their comfort zone, it’s pretty much a choose-your-own adventure. As such, tracks are important. They provide guidance in a choice full system.
Additionally, labeling yourself as pre-professional allows you to connect with other people who have the same interests. There are many clubs dedicated to supporting students interested in these career tracks. It's just a good way to bond with people.
The great thing about Duke is that it’s big enough for you to find a place for yourself, wherever it is that you belong. As common with larger schools, there is a strong party culture for those who want it. Party culture is largely dominated by Greek life or Duke’s SLG’s (selective living groups). However, many people who have experienced Greek life at larger institutions have shared that Duke Greek life is no where near as intense as at public schools.
I am not a fan of Greek Life myself (source: GreekRank). Even so, it is much tamer at Duke. Many people form strong friendships in Greek life and gain a lot out of it. There are Greek organizations for the wealthy and traditionally attractive, and ones that are for more diverse groups of people. You have a good shot of getting into a sorority over an SLG (defined below).
Greek life is not the only opinion for students looking for social groups. We have SLG’s which are organizations that people can join based on their interests. They are seen as the “chiller” alternative to Greek Life, and are often co-ed. We have SLG’s that are more traditional in party culture (examples: Wayne Manor, which is a frat in all but name; Cooper; Maxwell; Brownstone) and one’s that are more nerdy, serve underrepresented groups, and are slightly less selective (Nexus - largely LGBTQ; Round table - the cool nerds; Jam - nerds who party hard; Ileria - empowered women who also party hard; Fusion - Asian culture interest group; and more recently, a black culture interest group that is starting next year! Note: there are many more SLG’s not mentioned here).
There are also LLC’s – living learning communities – that are almost like an off shoot of Duke’s focus program (mentioned in an answer above). You can live with students with similar intellectual interests, and there is a house course (a 1/2 credit course taught by other students) that gives students the opportunity to engage with new ideas and get to know each other. This is an increasingly popular, low pressure, non-selective option for the less out-going. There is an ethics LLC (the most established), a religion LLC, and a political LLC (favored by Duke’s male, libertarian population).
You’ve probably noticed a lot of talk about living groups. This is a byproduct of Duke’s housing system, where you have way too much choice, and the easiest thing to do is live with an organization of some kind. The housing system is undergoing reform at the moment, and many students are excited about it.
Many students identify with their freshman dorms and their freshman communities, long after freshman year (if this confuses you, this is a reference to Duke’s freshman only campus, East Campus). Duke has recently created a new housing option for sophomores, in which all freshman in a dorm are moved to West Campus as a unit, by default, unless an individual opts out to live with a chosen social group. I suspect this will make community building a lot easier for those not interested in joining social groups, but that is yet to be seen. I love forward to it!
Duke culture is pretty competitive, but not in a cut throat way. It’s mostly self-imposed pressure. It’s a byproduct of the type of student who applies and is accepted to Duke. All of my experiences with Duke students have been extremely collaborative. The desire to be impressive makes many students over commit themselves to extracurriculars and classes. We tend to pressure ourselves into studying hard and executing all of our responsibilities “perfectly.” I think this is true across all social groups at Duke (Don’t let anyone tell you a sorority girl/frat boy is dumb. They would have to be blind to say that).
There are a lot of discussions about “effortless perfection” and students are trying to build awareness around this pressure in the Duke community. I think this problem is not exclusive to Duke, but rather to all elite institutions.