If I'm being honest with myself, it's a hobby first and foremost. Single-player gaming is a bonus. I love gaming but it's not really my MO at all. If I have something to game on, I'll game a bunch. If I don't, I'll find other things to do. I also do a lot of photo editing, as well as recording/mixing. Of course it's also nice to have a full monitor to look at instead of a phone/tablet/laptop display, and have a snappy machine that makes everyday tasks easy and helps me stay organized. It's also the heart of all of my entertainment. I don't have cable, no TV, even. I just find a PC to be better and more flexible than a traditional system. You have to mess with them more to get things working and there's upkeep, so they're not as convenient. But you can really tailor them to do whatever you want. And with everything being on the internet it just seems like the most logical be-all, end-all entertainment platform.
I definitely built this system expecting all of these things of it. But really, I just enjoy learning about, handling, tinkering with, and collecting PC hardware. I'd be lying if I said I NEEDED my 3900x
I have a ton a ton A TON of interests that come and go. I'm one of those people where it's a new series of wild info dumps every month, whenever you get me talking about what I'm up to. It's either not on my radar or fully zoomed-in. I don't have side-interests. I put my full energy into one or two at a time, jumping across a wide range over the years. And I can be extremely deeply into it for a while, but most of those are fly-by-night passions. And usually once I'm done with those things, I'm done. PC building just sticks with me... something about it has always got me going, since I was a kid... so I'm happy to invest good chunks of time/money into it. Aside from PC building, the only two things I can call 'lifelong passions' are music/guitar and writing. I move impulsively towards those 3 things, even if I drift to other things at times. Those are my mainstays. If I had a bunch of disposable income, I promise you I'd be one of those guys with rooms of PC parts... just doing builds, putting them to the test, modding them, upgrading them... documenting every endeavor with photos and specifics about them. I'd continue as long as time and resources allowed me to, long after my actual PC needs were met.
I always expect that my build will do what I need it to do, and more. So from there, it's all about the journey. Sometimes I'll pick up new endeavors just because my hardware can do it. For instance, I was never super interested in crunching or the plethora of things that can be done with VM's, but now I'm studying and playing with things like that, just because I have a CPU that's super-down for it (though a memory upgrade is going to be inevitable.) And it's teaching me all sorts of things that I never would've known about otherwise. I didn't buy it explicitly for those things. I bought it because I could and the technology that went into it is fascinating to me. And I do things with it that it's good at, because I now have something with those capabilities. And in the process, I learn more and deepen my appreciation for the whole deal. The more I get hands-on with different hardware, the more into it I get. So I keep doing it, even that's all it does for me. Who knows? Maybe I'll grab blender and start teaching myself about 3D rendering. That'd be fun for a while. This is how I think and operate when it comes to PC's.
That's the other thing... so many of my interests revolve around PC's in some way. It's hard to imagine me giving up nice hardware just because I haven't found a use for it. There's always a chance something is around the corner that the better hardware grants more entry to.
I guess for me, building PC's is sort of a ticket to learning more about them. Most of the hardware I tinker with is cheap or salvaged, just because I want something to gut and mess with. But every now and then I also like to go in on a build, just to make something that's truly nice and just sort of lose myself in the whole process. And then in the end, you can look at your creation and bask in the knowledge that you did well and put your mark on something that other people can see and appreciate as being meaningful in some way. It's basically a tangible manifestation for how I feel about PC's and tech in general. You know when you see my build that those things are a big part of my life.
And of course, a lot of people won't admit this, but I like showing it off. I mean, you put all of this time and money into a capable, nice-looking machine. You set it up exactly how you want it and go down to minutia getting this and that just right. Why wouldn't you show it off? It's a tangible result of something you probably love doing and care about. It's a way of showing people a piece of who you are and what you're about.
TLDR: I-unno. It's fun.
My HTPC does the same, but my main rig is faster, and this has more love in it 
I mean cable management, custom loop watercooling etc.. 
