This is probably basic question for you, but I'm not familiar in this area.

One of the questions in our final exam will be "Explain the need of A/D and D/A converters in digital controllers".

I could nowhere find the reason why "Plant" part of block diagram requires continuous signal. Isn't it supposed to receive digital signal and output digital signal? What inside process needs analogue signal and why? This lack of understanding is probably because I do not understand what Plant is for at all.

I also do not understand why A/D converter is not immediately after the Plant block, but only when it returns back to start of block diagram. Isn't digital controller supposed to output discrete signal?

I will be happy if you could explain this to me or point me to somewhere where it is explained for beginners.

Here is general block diagram of digital control system ^[1]: