> Why would someone who is developing their software on an x86 box with Linux care about ... Solaris?

There was a time when ZFS would have been a good answer for this. Of course, now it's available on FreeBSD and Linux.

At one point also there was zones, but again, containerization is pretty standard elsewhere now.

I guess also if you go further back, like the Linux 2.6 era, which is close to when Solaris was open sourced ... the 2.6 release involved a concerted effort to improve SMP support. So maybe you'd be interested more in commercial Unix in the 2.4 era, for better SMP. But again, Linux surpassed this ~20 years ago.