Caveat - run a Unix machine you don't care about (i.e. a disposable VM) out of disk space.
It's much easier to recover a machine if the storage is segmented. Yeah, a single partition is more convenient, but multiple partitions are more resilient.
Two of the reasons in this era were prioritizing which filesystem you wanted to overflow first, and because you necessarily used network mounts for lots of stuff (home dirs, project dirs, GNU tools, expensive software), even on workstations.
Sun was hugely into NFS, which was both a blessing and a curse -- you'd spend 4-5 figures each on a fleet of workstations with only 105 MB local drives, and they'd tend to all end up hanging frequently, because of a server burp or network burp. It didn't even have to be a burp on a filesystem you were using -- it only had to be (stale) mounted.
(I'm not this old, but I had the excessively good fortune to have access to super-cool computers and Internet as a kid.)
It really depends on what you're doing. On a desktop/laptop/workstation 1 partition is fine. On servers, all the data is not necessarily on the same disk. It's common to have the OS itself running on local disks and have everything else mounted on some non-local storage like a Storage Area Network, Network Attached Storage, NFS shares, etc.
It is a lot easier to upgrade an OS on a Unix box with proper partitions separating things you want to persist across installs from the OS. Makes backups and backup restores easier as well. It’s also best to have swap space be a partition. Though these days probably doesn’t matter.
A modern Windows install has something like 4 partitions these days for the c drive. Ones labeled restore, c, and I have yet to care to figure out the other two. On boot though you see just c.
Sorry about the delayed response. I misunderstood what you were getting at. It's because it's running Solaris and Solaris had different partition scheme compared operating systems now. There's wasn't really a "desktop" version of Solaris so it would just slice up the disk the same way it would on a single disk server. It's a really old school OS.