Real Time: what is its importance and significance?

Colleagues, since this section is for general discussions, I want to start my own, also very general. This is a discussion about the need to respect real-time parameters.

Once upon a time, I accepted the need to maintain real time as an axiom. I was choosing the most suitable motherboards, I was ran a program to evaluate the necessary parameters, and finally I found a computer with acceptable values. On this forum, somewhere, probably, my messages about this have been preserved.

And only now, I thought about the nature of this requirement.
Any movement of the CNC tools is carried out by stepper motors. While executing a program, the control unit of the CNC always gives the motor controllers one and only one command - take one more step. The direction can change, but the main thing does not change - the displacement quantum is one step of the drive motor controller. (It can be a microstep, but in this case it doesn't matter.) If the control unit orders to step the required number of times, then the instrument will inevitably come to the right point. Moreover, if in the process of moving the tool, having ordered to take an integer number of steps, the control post thinks for a minute, an hour or two, then this will only slow down the execution of the command, but will not affect the result in any way.
Because any working movement of the tool always consists of basic, quanta - movements of one step. And only their number and direction is important, not the time intervals between them.

Why, then, is it necessary to ensure the operation of the program of the control unit (in our case, a computer with LinuxCNC) in real time? What is the technical meaning of this requirement?

Ogogon. 

Imagine that your step motor driven machine is moving an axis at full speed
so the control is generating step pulses at a high continuous rate.

What happens now if the controller decides to sleep for even a fraction of a second?
The step motor (servo , whatever) stops getting pulses but cannot stop instantly
because you have requested an impossible deceleration (full speed to 0 instantly).
Basically meaning you have lost control of the axis position.

Ah because chipload and cutting the workpiece matters
if you have a tool spinning in contact with the workpiece, but not moving forward, you will rub. This is bad for tool life. In wood, rub long enough (which is not very long at all in human time perspective) and you'll burn the wood.
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Plus all the other reasons mentioned by others