I sense an undercurrent of incredulity here. The AR-15 accessory market shares some similarities to the after market PC market. The kind of people who play dress-up with their guns remind me a lot of the people who customize their workstations. Sometimes it's for a "need", sometimes it's for a "personal aesthetic" and sometimes it's just a hobby.
One of these days I'll go through the process of getting a threaded barrel and suppressor for my AR so I can shoot on my rural land without ear protection. You need subsonic ammo to truly make it ear safe, and 5.56 is kind of a bad sub sonic round (not enough mass, has a tendency to key hole on earth), so maybe I get a .300BLK upper receiver.That means a new bolt carrier group. Maybe I'll get one of those slick nitrile ones, or the bronze coated one so it looks cooler.. I already have a 1911, so I'll probably get a .45ACP upper instead of the .300BLK. Ammo is also cheaper.
Maybe I get rich and buy a drop in auto sear so I can turn ammo into noise. Maybe I'm a weirdo nerd so I buy one of those rail mounted chainsaws to help me trim the hedges. I like the color purple so I'm probably going to get one of those acrylic transparent lower receivers. Those are cool. Giselle triggers are nice.
There's different barrel lengths for different applications. A home defense rifle might have a shorter barrel, maybe a folding stock, tactical light and a red dot sight. Do I use a holographic sight like an eotech, or do I use an aim-point? Aimpoints can last like eons on a single battery, but eotechs are really easy to use.. I could leave an aim-point ON next to my bed for 5 years and it wouldn't run out of juice. That's one less button to press if I needed to use it in the night.
I like a bipod, longer barrel, and a 2x optic if I want to use it for plinking. What style of sling do I get, if any? Single point, double point? Probably depends on my barrel length and what I've practiced with.
Maybe I need special magazines because of preferences or local ordinances. We could debate direct impingement vs. piston gas systems like you might hear two people discussing intel vs. AMD.
I've got these small boulders that have really been bugging me. I've hit them with a sledgehammer for a few hours, but they're indestructible! I could buy a rock drill, or maybe masonry disks for my angle grinder.. I'm trying to use this "problem" to justify purchasing a .50BMG upper receiver. Wholly impractical, yeah.. but I could blow up my boulders from the comfort of my deck a mile away. Kaboom!
If you're left handed/left-eye dominant you might want the ejection port on the left side and an ambidextrous safety. Once cool thing about the AR is that the charging handle is ambi by design.
I could go on, but does this add any context to what someone's "needs" might be?