The single most important benefit is that if you want to install Windows 8.1 or 10 with the system in UEFI mode, you HAVE to have a GPT partition. It's highly recommended that you not install either OS on a system with an MBR partition as there is little reason to not do so. If you have an OS that currently uses an MBR partition, it's advisable to do an upgrade of that OS, back up all your important data and then change your BIOS settings and do a clean install of Windows 10 with UEFI mode enabled in the BIOS.
Once you've done an upgrade of Windows 10 your BIOS has been registered with the Microsoft activation servers, no further activation on your part is required after the clean install and you will have a much less problematic transition to Windows 10 as those having done clean installs have eliminated all the garbage from the previous OS interfering with the new OS.