Are all elementary particles consequential? Apologies if the title is misleading. Really what I'm curious about is if every elementary has a point, at least in what we know. I know the different types of quarks are important of course, as they make up protons and neutrons, along with electrons. However, what about leptons and antileptons? Or antiquarks, taus/tau neutrinos, muons/muon neutrinos? Do they do anything outside of their gravitational and weak force interactions? Do they make up any  composite particles that in turn do anything? They all clearly exist, but looking online, I can't find anything on if their actions are anything besides obeying the laws of nature. On  wikipedia the quote "The eventual recognition of the muon as a simple "heavy electron", with no role at all in the nuclear interaction, seemed so incongruous and surprising at the time, that Nobel laureate I. I. Rabi famously quipped, "Who ordered that?"" is also present which seems to point that they really don't do anything besides exist.
If I'm wrong in this assumption and any of these particles do more than that, please do correct me, or are quarks and electrons pretty much all we need for the world to function, with the rest being largely just extra bits with some exceptions here and there?
 A: In the low energy limit (i.e., the universe inhabited by humans), every particle we detect has a distinct and essential "purpose". In this realm we can temporarily create particles in our accelerators that only persist at higher energies, and these have no purpose in the low-energy scheme of things.
However, at early times when the universe was far hotter and denser, those particles were major players- and today we frolic in a soup of their decay products. So even those evanescent particles were essential- but at an earlier time in the universe's evolution.
A: Yes, surprisingly, particles rarely produced do have a point by their mere existence, undergirding the logic and consistency of the Standard Model, an open ended overly modest dubbing of the tight Theory describing them.
Even if you believed particles such as the τ lepton (rarely produced in accelerators: less than a gram of those has or will ever be produced in them) had no role to play, consistency of the theory requires quarks of that generation to exist. Technically, these are required for gauge chiral anomalies to cancel, which is required by the consistency of the gauge theories involved. The standard model would be inconsistent without them.
So confident were physicists of the existence of the b and t quark before their discovery, because of this, that their discovery brought relief, but not excitement; it was not a game changer.
In turn, the third generation of $b, t; \tau,\nu_\tau$ is also required, as, without it, CP violation, a crucial property of nature would be very hard indeed to explain.
And so on... The Standard Model is a wondrous puzzle that fit. If a crucial piece is missing, you have every reason to expect it to be there, sight unseen. The hell with what you see and the early history of the universe. The history of particle physics is finding things that just are, even if they don't perform obvious realistic functions act the time. They have a "point" more than our world and silly existence has a "point", cynics might think. Rabbi's  exclamation is one of wondrous excitement about something beautiful to be explained, not testiness.
Roles? They have more intricate roles than instruments in an orchestra. Marimbas, anyone?
