I agree that there are social messages that this is just what people in relationships do. For me, it's had several benefits though both of the adults I lived with started as a temporary measure (one a holiday, the other a place to stay after finishing university while he gets on his feet) that has ended up lasting a lot longer because it's worked for us even with the bumps and annoyances.
I sleep better and am more at ease with other adults I trust in the house. I didn't notice it until after we had a friend move in too, but adding in one more adult I could depend on if shit hit the fan overnight helped me sleep better and after some adjustment to another person, my anxiety levels dropped too. Looking back, I was the same as a teenager when I would be left for weeks at a time - every sound at night would grate my nerves and remind me of which windows didn't really lock properly or how easy it would be bust out a pane in the back door (very thin glass in a grid shape that came out more than once, went months with just cardboard in one of the holes once) to unlock it or incidents with intoxicated relatives that were often worst at night and they dropped after I left home and moved in with my now-spouse and had nice people around all the time. With other adults I trust and some improved security my brain can offload from hyper-vigilant mode and get some rest which makes everything else better.
I like that I can shift the physical and mental load and we can share skills - like we found out the heat pump in our dryer has died, possibly damaged in the recently move but it stopped collecting water. I arranged the engineer to come out as I had arranged the warranty on it, my spouse got the drying line and all the other things set up, we've all talked through options for the future. The friend we live with has done a lot of tasks over the last couple of months from painting to changing fuses to driving me to pick up a Facebook Marketplace keyboard because that got damaged in the move too and my DD1 needs one for lessons asap & I was so frazzled (and medically can't drive). This obviously can also be done with people not living together, but it somehow feels easier this way and it never feels like an imbalance like it does with some other people in out social circle where they've done several things for us and being people who never ask for help I've no idea how to redress that imbalance with them.
It can be annoying - I live with 2 other adults plus 2 teens and 2 younger children and very little of what is around is mine, very little of the mess comes from me, my friend-lodger particularly confuses me with his logic sometimes (he's very very neat in some areas and downright disgusting in other areas and I do not get it though have contented myself to at least most of his slobbishness being in his room that I don't need to see but I've had to have a chat with him and the kids about the sink not being a garbage disposal again).
Even with all those bugbears of people being complicated and having way more stuff than me, I prefer communal living. Maybe it's in part because I was pushed to live essentially on my own far too young and I've had so many systems then and since that let me down when I've tried to go it alone, or maybe it's just me. I like that when things go wrong, I've two other adults I can get support from at least and often also get practical help at pretty much any time with minimal niggling that I'm being a bother because I know I can and do give them the same in return in my own way. I like when things are going right, our home is a built in party to celebrate. It helps me be more independent in a way as happened this week where I had a fall, there was someone right there who checked on me and helped me with the task I'd been in the middle of, and after a bit, I went on with my day. If I'd been alone, I would have been more likely to do less as I'd have been too scared of another fall and become risk-avoidant whereas with other adults in the house I'm not as much.
Even little things like finding that annoying task that I've been procrastinating has been done because I'd mentioned it in passing and one of the other two or even one of my kids decided to do or doing that for someone else is nice.