What useful thing did you conduct with it?
You store constants and your actual program code in Flash. The limitation of Flash memory is that you can only erase a whole page state time. On many of the smaller microcontrollers, one page may be larger than your SRAM!
So you treat Flash as read-only, writing it with a separate hardware programmer. You store state in the limited SRAM. You would also read or modify external state by loading or writing to special function registers outside of the Flash and RAM address space, where the "memory" mapped to that bit or word would do something in hardware. It would tell you whether a digital pin was on or off, or turn it on or off, or tell you the analog voltage on an input pin, or configure the pulse width of a pulse-width modulation timer, or read/write to a communication port, or set up a hardware interrupt timer.
Here's probably the most common example of this class: http://ww1.microchip.com/downloads/en/DeviceDoc/40001239F.pd...
They're often used to 'glue' two separate circuits together. For example, you could use it to measure an input, and in one mode output a high voltage if that input was on, or in another mode output a low voltage. In more physical examples, you could read the position of an analog input knob and control the pulse width of a TRIAC to control the speed of a motor. You could monitor the capacitance of a touch sensor (with an ATTiny) and turn a device on or to standby, or do a hard reset if the button is held down for a long time. You could modulate an RGB LED through a rainbow of colors. You could run a flashlight in a high power, low power, or strobe mode depending on how a button was pressed. And so on.
If it would have had RAM, one could have used it to implement a memory or to implement a 'normal' infix UI.
And no, it isn't cheating that it uses a CPU designed for calculators. At best, that keeps the ROM space needed down, but I doubt that, given its somewhat weird instruction set.