Why is /tmp a symlink to /private/tmp?

As I understand it, it's a holdover from NextStep (which OS X is based on), and NextStep did it to support NetBooting. The idea was that you could boot from a network-hosted volume (probably read-only, and certainly shared with other computers), and early in the boot process mount a local (writable) volume on /private; as g mentioned, this allowed runtime-modification of /var and /tmp, as well as per-computer settings in /etc.

This isn't needed anymore, as Apple's current NetBoot system uses a shadow disk image to store changes anywhere on the boot volume. But some programs/docs/etc now assume the files live under /private, so it'd be too much trouble to switch them back...

Update: since I wrote this, Apple has stopped supporting NetBoot, so the original purpose of /private is even more obsolete. However, in macOS Catalina (version 10.15), they've added a new volume split. In this case it's for security rather than to support NetBoot, but it works in a fairly similar way.

Catalina's system volume is mounted read-only, with a read-write volume mounted at /System/Library/Data (analogous to the old system that mounted a RW volume at /private), and "firmlinks" making parts of the RW volume appear at their usual locations in the filesystem (again, analogous to the symbolic links that make parts of /private appear at their usual locations). For example, /Users is now a firmlink to /System/Library/Data/Users. The Eclectic Light Company has a good summary.

Catalina also still has the symbolic links to /private; thus, when you access /etc on Catalina, it follows the symlink to /private/etc, and then the firmlink to /System/Library/Data/private/etc

I have always wondered the same thing. I can't find any documentation to support it, but typically this pattern is used to make it easier to store files on another volume (e.g. hard drive). This allows the drive to be mounted (e.g. attached) into the file system in one place. For example when the drive is mounted at /private and then the folders /etc, /tmp, and /var are then all located on that other drive.

What I can't say is why this would be beneficial. It is worth noting, however, that these three folders contain "data" files such as configuration, temporary, log, transient and database files rather than executable code contained in the /bin, /sbin and /usr folders.

Not sure about the historical reason, but OS X always “reorganized” the typical Unix structure. /tmp is not the only thing that goes to /private, it also has /etc and /var.

Maybe someone with more OS X background can come up with something more reasonable.