Everyone knows about the BIOS screen that shows up before your operating system starts to load. BIOS is a small program stored in a small chip on your computer’s motherboard and manages how the different hardware interfaces with each other before the operating system takes over. Modern computers now have UEFI which is similar to BIOS but offers much more advanced features like integration with the operating system itself. If you have UEFI enabled, then you can change the UEFI settings right from inside the operating system like Windows or Linux. This is simply not possible with the older legacy BIOS where you will have to restart your computer, press a special key like Del to enter into the BIOS mode and then change the settings.
If you have bought a new computer on which no operating system is installed (many notebook computer manufacturers are making these available), then you should first make sure that UEFI is enabled before installing Windows 10. Because by default the OEM have the legacy mode enabled to make it easier for everyone to install the operating system. But if you install Windows in the legacy mode, it will create MBR partitions as opposed to the GPT primary partitions which are needed in the UEFI mode.
One way to make sure that you will be able using the UEFI mode is to disable the legacy mode in your computer. For this, you will have to press F2 to enter the BIOS mode when the computer starts to boot and then make sure that under the Boot section, you have the UEFI boot mode enabled.
Obviously this differs from one computer to another, for example, in some computers you can choose the boot mode (UEFI or legacy) for all the devices. But in some other computers, you have to choose these modes for each of the individual devices. You may have to consult the documentation that came with your computer’s motherboard for more details.
Conclusion: Enabling UEFI boot mode before installing Windows 10 makes sure that your partitions are GPT and that you can access UEFI from within Windows.