Hi,
if you don’t have a UEFI based system (Windows 8 or 10) installed, there isn’t any real need for it as you are most likely already booting from “Legacy BIOS” and UEFI is indeed switched off!. So Ubuntu doesn’t really need to worry about whether or not the PC is UEFI compliant!. 
Case when Ubuntu must be installed in UEFI mode
Having a PC with UEFI firmware does not mean that you need to install Ubuntu in UEFI mode. What is important is below:
if the other systems (Windows Vista/7/8, GNU/Linux…) of your computer are installed in UEFI mode, then you must install Ubuntu in UEFI mode too.
if the other systems (Windows, GNU/Linux…) of your computer are installed in Legacy (not-UEFI) mode, then you must install Ubuntu in Legacy mode too. Eg if your computer is old (<2010), is 32bits, or was sold with a pre-installed Windows XP.
if Ubuntu is the only operating system on your computer, then it does not matter whether you install Ubuntu in UEFI mode or not.