I'm no expert as I've only really begun exploration (141kly, max 38kly from start), but here's what I've learned through the community and from my own personal experience...
Ask 10 different CMDRs and you'll get 10 different answers.
It depends on your perspective. Are you going to find new life forms, totally new planet types, a new alien race, etc.? Not unless FDev decides to create some more. I think the odds they have created some and we haven't found them yet are extremely slim.
However, what about finding a binary system with three black holes, six ELWs each with an ELM? The uniqueness of discoveries have more to do with system composition rather than anything else.
Well, some people do enjoy that. Also, you might be able to see something through a telescope if it's extremely close (relatively speaking), but you can't orbit it. And you can't go see what's in a nebula. You can't see Sgr A*. You can 't go drive on it in a dune buggy.
I couldn't disagree more. The game excels in many areas.
Here are some other reasons:
Credits
You can make some money at this, although the credits per hour is going to be extremely small in most cases. Heading to Sgr A* and back, for example, would probably bring in about 100M credits assuming one scanned everything along the way.
Achievements
Some people are into getting badges (at sites like EDSM or INARA) and will do it for this reason. Others want the 'Elite' status. Still more just want to gain the sense of accomplishment.
Bragging Rights
"Oh, you went to Colonia? Well, I went there and then to XA, Beagle Point,
Oevasy SG-Y d0, swung through the Andromeda galaxy, shook hands with God and then walked back to Sol."
That "Itch"
Once someone begins exploration, they feel drawn back into the black.
Community
Whilst exploration tends to be a solo gig, there is camaraderie between explorers. Also, participating in an expedition allows one to meet other CMDRs and socialize; even if the CMDR himself is alone in the void.
Relaxation
Flying through the void, scanning planets, driving around in the dune buggy... this is all pretty casual, relaxing stuff. Of course, that typically leads to complacency, which then leads to a rebuy and loss of all exploration data.
Chemical and Other Abuse
Explorers are a weird bunch and probably suffered childhood trauma and now drown their feelings with booze while they scream drunkenly at the screen "You're not even my real father!". Or, maybe that's just me.
Anyway, give it a shot. You might enjoy it. Then again, maybe you won't. I wondered the exact same thing you did, and now I'm kinda hooked. Having said that, once I start heading back to civilization, I do get antsy and want to get back. Then, I pretty much want to leave again.
Fly safe! o7