Dynamic disks were first introduced with Windows 2000 and provide features that basic disks do not, such as the ability to create volumes that span multiple disks spanned and striped volumes, and the ability to create fault tolerant volumes mirrored and RAID-5 volumes.

 

Dynamic disks offer greater flexibility for volume management because they use a database to track information about dynamic volumes on the disk and about other dynamic disks in the computer. Because each dynamic disk in a computer stores a replica of the dynamic disk database, Windows Server 2003 can repair a corrupted database on one dynamic disk by using the database on another dynamic disk.

via What Are Dynamic Disks and Volumes?: Storage Services.

I am unsure as to why this is done at the OS level.