The situation with Steam Machines

On Steam, I accidentally landed on the page of these units. I used to hear about them, but in general I didn’t quite understand their purpose and the future, so to speak, the diversity of this product. Explain in more detail the very essence of these consoles from Steam. Are they replacing the PC? Turn a PC into a console? If you buy a cheap steambox, does it mean that it will have games on medium / low settings, and on an expensive steambox it will be on ults? And if the console becomes obsolete, then you need to throw it away and buy a new one? Then what's the difference from a regular PC?