It makes your system somewhat faster... especially with SCSI hard drives. I know this use to be required in 2K/XP if you wanted your SCSI drive to perform up to speed.
You can convert your drives to dynamic disk without losing data. However, you won't be able to boot into any other operating systems and in order to go back to a basic disk you'd have to format. You also can't convert (most) laptop hard drives to dynamic disks. And as mentioned above you can also use dynamic disks to take advanatage of software RAID.