QEMU is an emulator, that is, it is a software which copies the behaviour of a complete computer system. Why is the ability to pretend to be another system a valuable capability?

• With the passage of time, computer systems advance
• Sooner or later, a particular mission critical software you use is no longer supported
• Perhaps there is no source code for this mission critical software
• Perhaps the operating system is defunct
• Finally, the hardware itself does not exist anymore

For example

• What if your program was hard coded to address hardware registers which no longer exist on newer PCs?
• What if your program can only run on an obsolete operating system such as Windows NT4?
• What if you can no longer source PCs with the necessary hardware registers?

Perhaps you may be able to run your program in Windows NT4 in a QEMU emulated ancient hardware.

Let's try running Windows NT4 on a modern multiprocessor system!

Here's NT4 booting...

NT4 is unable to get past system configuration detection...

Now we emulate an obsolete Pentium Pro system, and NT4 is starting...

We can now logon to a virtual NT4...

The ancient 1996 desktop...

Finally, we can do a proper shutdown of our virtual NT4...

When you really need to continue running legacy mission critical software on an old and dying hardware, look to machine emulation to solve your problems.