While modern PCs do ship with UEFI, most of them have a legacy BIOS mode for using the hardware that might not be fully compatible with UEFI. As per the reports, after a few years, this option might not be available at all.
Intel has announced that it’s planning to end the support for legacy BIOS compatibility by 2020. According to Phoronix, Intel’s Brian Richardson hinted at the same during a recent presentation.
By 2020, Intel will be only supporting UEFI Class 3, which means no legacy support of CSM compatibility.
It’s worth noting that this change doesn’t mean that “Secure Boot” will be mandatory. So, it won’t be preventing you from running an unsigned operating system on an Intel-powered machine. This requirement of mandatory secure boot comes with another category called UEFI Class 3+.
With this change, there would be advantages like smaller code size, smaller validation/support footprint, usage of new technologies, etc.
You can read further about this change in this presentation (PDF).