That is basically it. The chance to run into someone, and the possibility of that either being a friendly, hostile or neutral encounter. It adds some danger, surprise and the need to do some basic preparations, planning, and develop some skill and awareness. Some like it, some don't and some apparently have been deeply traumatized by it and can't really accept the above.
Surprise?
Dunno, haven't seen much of a surprise in thousands of hours in Open.
Mildly Player-Populated System : 99.9% mutual ignorance, 0.1% "o7" which may or not get a reply
Hotspot : 90.0% GvP (absolutely no surprises), 9.9% mutual ignorance or "o7" which may or not get a reply, 0.1% short chat while sitting on the pad
Average standard System : 100.0% Nobody there anyway, you're alone
Given the digital nature of the few "means of interaction" the Game offers, not much benefit unless you seek to test your Ship, its Engineering build, its Defenses or Offenses or tactics. PewPew nor nothing at all.
Might be just me, but just sitting in FSS Screen alone after System entry and being a totally blind sitting duck... Meh.
Staring mostly at the Network Bandwidth and having to leave FSS to assess immediately upon a Net spike isn't for me. Did it but found it extremely annoying i.e. when chasing HGEs.
(just on that example, there's only downsides to Player presence when busy chasing Signal Sources and zero benefit other than painting a "I'm clueless please shoot me" on yourself wile in FSS).
Frankly, I find that offered mix boring.
The old days where there was still a thing called "Piracy" and when mindless Ganking in Meta builds going predictable like clockwork wasn't a thing yet... that was probably the best time I remember in Open.
(other than being "born" in Eranin and the 1st Courier Mission to an Outpost in the Styx System showed me 25+ Harmless Sidewinders frantically trying to dock lol.... I'll still remember that sight in 10 years from now

)
Now those were
awesome times in Open. Too bad they're long gone.