Advantage of GPT disk.

What is the advantage of GPT disk?
GPT disks can grow to a very large size. Break the 2TB limit of MBR disk. GPT disks allow an almost unlimited number of partitions. Each GPT partition has a unique identification GUID and a partition content type. And you can have more primary partitions. However, the Windows implementation restricts this to 128 partitions.

What is the limitation of MBR disk?
MBR disk only support up to 2TB partition size and support only four primary partitions. If more partitions are wanted, you have to build a secondary structure known as extend partition, and then you can create logical partitions. MBR partitioning rules are complex and poorly specified.

Why you need convert MBR/GPT?

As known that MBR disk only support up to 2TB partition size, so when you have a disk which is MBR formatted and larger than 2TB, you could only use 2TB disk size among it, but the rest of it could not be used. Convert MBR disk to GPT disk is considerable necessary to make full use of the disk space. But note that the program prevent you from converting system disk, since GPT system disk only could boot from UEFI motherboard, but MBR system disk could not boot from this configuration.

In addition, since Windows 95/98/ME, Windows NT4, Windows 2000, Windows 2003 32-bit, Windows XP 32-bit version does not support GPT partitions. A GPT disk will display as GPT protective in these systems, thus you could not access the data until you convert them back to MBR disk. But note if you want to convert any larger than 2TB GPT disk back to MBR disk, you'd ensure that the data does not account for space more than 2TB.

see also>> How to convert GPT/MBR
http://miratouch.blogspot.com/2013/10/how-to-convert-gpt-disk-to-mbr-disk.html