The status byte is for people who want a software-based method of starting the bootloader. For example perhaps their firmware accepts a special command over the UART that when received sets the status byte and then causes a reset.
The advantage of this method is that it doesn't require manipulating pins, i.e. having the end user press buttons. Therefore, going back to your original question, if you have only the minimum pins in your hardware you can use this method to get into bootloader mode. Clearing the status byte and reset gets you back out of it. So you can go back and forth. The critical point is that a blank device - i.e. new from the factory - resets into the bootloader. So the very first time you program it you include support for this method.
The disadvantage is that if your firmware doesn't run for some reason then it won't work.
