The question on the table can be divided into two parts. First does your computer need cooled? And how cool should it be??
Answering the first question is easy. Yes, your computer needs to be cooled. Processors have gotten very efficient at converting electricity into work but still generate heat as a byproduct. Something about equalizing an equation that contains an efficiency coefficient. Lots of math and the bottom line is the chips need to run at a certain temperature to maintain that efficiency.
Answering the second part is a little more complicated. How “cool” should your computer be? Hardware manufacturers like Intel, AMD and NVIDIA go through great lengths to answer that question. They want to know how long these components will last and ultimately determine if it will survive the entire warranty period. Beyond that they are no longer responsible and will move on.
I’m sure there are other reasons like preventing a meltdown and ensuring every calculation is fast and accurate and if the product will be competitive but, if you consider the economics the chip maker cannot make money if their products cannot deliver what is promised and/or are retuned. This is why OEM coolers were invented and deliver just enough cooling to satisfy the above requirements.
If you look at your cooling solution from the above perspective the answer is “No” there is no reason EVER that you would need to watercool your system.
However, that is not why you are here.
You are here looking for justification for doing maybe one of the most exciting things EVER to your PC. Watercooling will not only cost more than twice the original cost of your computer but will have the potential to increase performance, lower noise, increase maintenance efforts and generally look badass. (That is a technical term for the most awesome thing ever)