In no particular order:

I learned a lot of uncomfortable stuff about my behavior in a short time

These were things that I had never noticed before, despite being a very introspective, reflective person (and a psychologist to boot). And I think the relationship was key to that. While my friendships also uncover new truths about myself, I can generally keep things as superficial as I desire, and I have a lot of control over when to see my friends, in what context, and for how long. Relationships (generally) have a tendency to break out of those patterns, uncovering a lot in a short time.

This has proved so beneficial that I don't even regret some of the truly toxic and emotionally harrowing situations I've been in. No pain, no gain, I guess.

For certain things of varying importance, I could rely on my 'significant other'. A form of symbiosis, I guess

I am not a practical or very, uh, physical man. When I'm caught up in something, I forget to eat (or eat healthily), I forget to do my laundry or clean my house, I forget to meet friends (either for my benefit, theirs, or both of ours), or I forget to even just go outside.

And once I break out of this obsession, I often realize in hindsight that it wasn't always worth forgetting that other stuff. And yet I keep doing this, and from looking at my father, this is quite possibly something I cannot break out of without significant effort. Effort that I'd rather spend on other things.

When I was in a relationship with someone who was different enough in these regards, but similar enough to not cause too much friction, my life improved immensely, including the efficacy of the targets of my obsessions. It turns out a balanced life is usually better :).

While this might be an extreme example and you might not identify with this, I think most people have their blind spots, and while my previous point could arguably be 'solved' by fixing the flaws, it is much harder (and perhaps less desirable) to 'fix' these personality traits.

Sex, physical intimacy, and a direct 'responsibility' to another person is generally a fundamental aspect of being human

I am happy alone. Truly happy. I also probably have a lower libido than most. I don't understand people who can't be alone or need sex all the time and will put up with terrible partners or consequences to achieve this.

And yet, I learned that while I am perhaps unusual in this regard, it's quite likely that to some degree I learned to be happy alone, and I learned to be happy without having sex very often, but this might not be my ideal state. And experimenting in this regard has given me a more nuanced view of myself, where I realize that the hug I get might be exactly the one I needed, even though I didn't feel like that before I got the hug (for example).

The bottom-line is that I've known many people in relationships and I've known many people without them. Happiness is perhaps as closely related to being in one or the other category as we often think, but there is a relation, in my opinion.

'Norms' can be at the very least informative as to what is likely a healthy approach. And my personal experience with people who have no (romantic) relationships, or for that matter people who have no friends, is that especially later in life they seem to grow in a way that does not seem to make them happier. More successful perhaps, or more valuable to society, but often obviously (to me) less happy. Loneliness is a terrible affliction, and can sneak up on you.

Whether you want to take the risk of being 'on your own' (in whichever particular form you choose) depends partly on your desired outcome, of course. Newton was a recluse, but perhaps he lived the most fulfilling and happy life he could have led, and at the very least he was very important to the world (in my opinion).

For me, I don't want to take such a risk without having experimented with the alternatives. I might very well choose to remain alone for much of my life, but at least I have (some) idea of what I might be missing out on, and some experience and skill to reconsider my choices later (which can be hard if you have no experience in this regard).

That said, I'm still young and I might be full of shit... :-). Ask me again in a few decades!

(also, perhaps the key point is that I didn't always greatly desire a relationship and I didn't always like the person. Especially the first time, it was a conscious choice to try something big, new, and potentially dangerous, but less dangerous than I had initially thought).