There was also a "server" model Mini but it was very short lived and was basically a regular Mini with the "Server" software pre-installed, something that you could just throw in via the App Store with one click anyway.

It had a five year run and saw four different hardware models. It included two hard drives instead of either one hard drive and an optical drive, or just one hard drive (after they ditched ODDs).

Mac OS X Server was its own operating system originally. It was still the same core OS, but had a ton of additional servers built in. Non-exhaustively, they included IPSec VPN, email, calendaring, wiki, SMB and AFS file shares (including support to act as a Time Machine backup destination), LDAP, DNS, and software update caching before it came to macOS proper. The Server app released via the App Store was a shadow of Mac OS X Server.

These were quite popular in small professional offices like law firms.

I saw it save the ass of a client who had one, as they got robbed and all their desktop computers, mostly iMacs, were stolen. The Mini was as much as lost in the wiring in the network closet and was overlooked, so everything had backups.

They actually had a Mac Mini Server as well for a bit. It made sense because it had a second hard drive instead of an optical drive and came with a Mac OS X server, back when that was a standalone $499 product: https://support.apple.com/kb/SP586

(Not sure what differentiates the later model Mac Mini Servers from the regular Mac Minis, since Mac OS X Server just became a $19 App Store purchase, and optical drives were no longer a thing in Mac Minis)

I have one of the 2009 Mac mini Servers running Ubuntu 22.04 LTS like a champ. It’s still a great machine. Upgrading the HDDs to SSDs was a bit of a chore, but doable.

They discontinued the Mac mini Server line in October 2014, which was still sold with two drives instead of one. Configurable to order with SSDs by that time.