I've worked on a couple of systems that didn't have one... Typically, you'd have to BUY a hardware development system, a software development system, and maybe a separate programmer. That's usually a few-hundred dollars, and it could run into thousands. Oh... You also need a power supply for the development board.
I was AMAZED that I could buy an Arduino (and nothing else), and in about 15 minutes I had compiled, uploaded, and was running the "blink LED" example. That process usually takes about a day!
Why the boat loader ?
It saves a boat load of time, trouble, and frustration.