The Master boot record is created when you first install Windows, on the first partition you create. If the MBR is damaged, you won’t be able to boot into Windows.All Windows versions – Windows XP, Vista, 7 or 8 – use MBR and, if damaged you won’t be able to boot.
Besides this feature, it enables you to resize partition, merge partitions, make bootable CD, allocated free space, migrate OS to SSD, and more useful features.
However, some operating systems such as Windows XP 32-bit, Windows 2003 32-bit, Windows 2000, Windows NT4 don't support GPT disk.
In these operating systems, GPT disk will be shown as GPT protective, and you are not allowed to access data in the disk unless you convert GPT to MBR. So among the large number of Windows 10 users, they need to convert MBR to GPT disk or convert GPT to MBR disk depending on their own requirements. Input "clean" and press "Enter" to delete all partitions or volumes on the selected disk. Input "convert mbr" and press "Enter" to complete the conversion from GPT to MBR format.
Note: To guarantee PC bootable after the system/boot disk has been converted from MBR to GPT, you will need to enter into UEFI firmware settings, and switch from Legacy BIOS to UEFI mode.
You may need to check your PC's or motherboard's manual for its specific instructions on how to change this setting.