Honestly, there's no one single reason. Every time I'm asked, my answer changes a bit. I started with them because of eggs, but my reasons grew. I keep them for meat, companionship, breeding, showing, and yes, eggs. I admit freely to playing favourites with my flock—a few of them are treasured pets that will live out their lives here, and some are hatched with the intention of butchering.
I love how chicken keeping can be as simple or as complex as I want. There are so many facets of their care and physiology that I will never run out of things to research and experiment with. I love hatching them. I love seeing them grow up and explore the world. Social dynamics of the flock are fascinating too and a few hours a day of watching them will yield much insight. You can hyperfocus on, say, the exact mechanics of gas transfer in the lungs, or you can zoom all the way out to the history and domestication of the chicken.
As for the aspect of butchering—I wasn't ready to do it for a few years. I took my time, but once I was ready, I did it. Large amounts of technical research helped, 'cause I'm just that kind of person. Give me something simple and I'll turn it into a 10-page opus complete with diagrams, lol. Nobody can 'make' you ready. You have to come to your own conclusion, and if you decide you want to do it, that conclusion has to be firm so you don't wimp out once you've got the bird in the cone. Knowing how to cull your own birds is a necessary part of owning them, IMO. Even if you don't want to process them, you're going to want that skill when you have a terribly injured and suffering bird that has no hope of survival and is going to die anyway, only slower and in a more excruciating manner. I realize this might be a rather charged statement, but I believe that if you have a bird that is suffering and you watch them die slowly for your own feelings' sake, you are failing at the promise of kindness to them that you made when you took ownership. In the "wild", a predator would have killed them quickly once they started to fail. We protect them, so we have to take on that duty too.
I don't
enjoy butchering day. But it's a part of how I choose to run my flock. I hatch them, I give them a really great life full of grass and bugs to chase, and then they die quickly and provide food for me and my family. I don't turn into a huge, emotional process, I do it quietly, cleanly, and with awareness that this is a life that I am taking and I should not let it go to waste but rather to a continuation of my own. As for what happens when you get attached, well, that depends on what you've decided. Me, I try in general to have a greater share of logic when I decide who needs to go. I look at factors like egg production, health, age, and build. Emotions aren't a good way to run your life in general. There's still a few birds that I like more than the rest due to unique behaviours or colouring, and I keep them, because why not? An extra three birds won't break the bank. Chickens are a hobby, after all, even if I do try to make them pay mostly for themselves.
You could debate forever on the ethics of it, but I don't have the time or patience to do that over and over again when I have already hashed it out with myself, which is all that really matters. The bottom line is that you've got to be comfortable with the choice you make. There's nothing wrong with keeping them for pets, and there's nothing wrong with keeping them for meat.
Edit: I think the human concept of "living out a life" is kind of sketchy as a whole. I don't want to "live out my life" if it means I'm in a wheelchair in a nursing home, rotting away in mind and body. I'd kill myself first. Humans have a fear of dying in a way no animal does—maybe we should learn from them and quit worrying about measuring time and start enjoying things like worms. Wait, not worms. That's ONLY for ducks.