Real mode provides no support for memory protection, multitasking, or code privilege levels.
Since a primary design specification of x86 microprocessors is that they are fully backward compatible with software written for all x86 chips before them, the 286 chip was made to start in 'real mode' – that is, in a mode which turned off the new memory protection features, so that it could run operating systems written for the 8086 and the 8088.
Windows 3.1 removed support for real mode, and it was the first mainstream operating environment which required at least an 80286 processor.
Some programs predating the 80286 were designed to take advantage of the wrap-around (modulo) memory addressing behavior, so the 80286 presented a problem for backward compatibility.
Forcing the 21st address line (the actual logic signal wire coming out of the chip) to a logic low, representing a zero, results in a modulo-2^20 effect to match the earlier processors' address arithmetic, but the 80286 has no internal capability to perform this function.
Most versions of the HIMEM.SYS extended memory driver for IBM-/MS-DOS famously displayed upon loading a message that they had installed an "A20 handler", a piece of software to control Gate-A20 and coordinate it to the needs of programs.
In protected mode the A20 line needs to be enabled, or else physical addressing errors will occur, likely leading to a system crash.
The changing towards the NT kernel resulted in the operating system not needing DOS to boot the computer as well as being unable to use it.
The only way of currently running DOS applications that require real mode from within newer versions of Windows is by using emulators such as DOSBox or x86 virtualization products.