Also OS level emulation. Linux system calls can be translated to Solaris system calls allowing to run Linux specific workloads without having to emulate hardware and a full Linux kernels. Furthermore before Dtrace was ported Linux it allowed to debug Linux workloads under DTrace.
Unless the software is "vendor locked" with linuxisms. I wish I could run everything only on zones but alas, still have to mix in some bhyves as well for e.g. docker support...
