It gives you more chance to meet people.
And ultimately the ED galaxy is supposed to have other living breathing independent pilots flying around in it.
That's what Open gives me. NPCs in Elite are purely window dressing. They are Disneyland animatronics. They actually
do nothing. They pretend to deliver cargo, but they don't. They pretend to pirate, but they don't - sure, they might steal some goods from you, but those goods and the pirate himself disappears from reality as soon as they jump out. They are phantoms, illusions, stage props conjured from the ether for a moment to give you something to look at, only to disappear back into that ether as soon as they leave your instance. Once you see this, the Elite feels very fake and empty, no matter how many NPCs the engine spawns up in your instance. At least it does to me.
Real players, on the other hand, are truly real. When I see a player landing in a cargo ship, I know they have real cargo that has come from somewhere, and that real credits and influence are being exchanged, making a real impact on the BGS. I know the CMDR him / herself has a real and unique personal story, a real goal, and a real course plotted through the galaxy. And I can learn more about those goals and history if they are willing to strike up a short conversation via the comms. Or I can just fire a quick "o7" and be on my way, still enjoying the fact that Elite is not just a Disneyland ride, but a living, breathing galaxy with actual people making actual impacts on that galaxy. This is the beauty and magic of Open in Elite for me.
Now I will say (again, for the 1000th time) that in a game like X4, where NPCs are much more "real", as in they always exist independent of me, I'm much more satisfied with the offline "solo" experience. I do wish X4's NPCs had a bit more personality, but at least every single one has a purpose and influence on the galaxy I'm a part of.