This has been answered several times already.
- those who need more than 16gb ram,
- those who need to use PCI cards (audio, video, SCSI,...),
- those who need raid scratch disks,
- those who prefer to use their existing matte screens,
- those who need best in class performance (our current Mac Pros being pretty far from the cutting edge), that includes more cores, faster GPUs used as co-processors,...