First and foremost it simulates the natural waters of many South American and Asian species.
Many hobbyists have more success breeding difficult species when kept in black water. It is believed that certain chemicals and compounds released by the peat and leaves signal the fish that it is time to breed. It softens and acidifies the water thereby simulating conditions at the time of year when breeding occurs. Egg fertility & hatch rates are increased while fry mortality is decreased.
Also, when you say that the first set of pics look good, that is the before pic. What you can't see, and it is very difficult to capture in pics, is that after adding the black water the gold on the fish practically glows. It enhances the colors of fish that come from these types of habitats. Cardinal tetras are beautiful in their own right but you would be amazed to see them in black water. The blue stripe which is normally bright, again nearly glows when in black water.
Until the next water change. It stays in the tank until you remove it either through water changes or chemical filtration.
If it were up to me, years ago. Since I really have no say in the matter I guess whenever they get around to it! LOL