There are 4 broad steps to creating a cup of coffee from start to finish:
There are entire books written about each step, and you could spend decades mastering any one of them. As you've probably guessed, we focus on roasting. We may discuss some of the other steps in the future, but there's a reason we're Roshi Roasters and not Roshi Growers, Brewers, or Processors. Roasting is what we know best.
Why roast at all?
After coffee is grown, picked, and processed, you are left with a hard greenish-gray seed. Nothing about it is appetizing, and if you tried to brew coffee with these "green beans" you'd be disappointed. The flavor and aroma of coffee that you love does not exist at all until the beans are roasted.
Exposing the beans to high temperatures (400+ degrees Fahrenheit) causes a number of interesting and complex chemical reactions to occur. This includes the Maillard Reaction, which is responsible for many other delicious flavors in foods (steak, french fries, toast, marshmallows... pretty much anything that browns as you cook it).
By controlling the duration/temperature of the roast, you can choose which reactions occur in order to bring different flavors out of the beans.
The finer points
Coffee comes in many varieties, grown in different countries, at different altitudes, in different soil, using different techniques. The roaster's job is to experiment and determine which "roast profile" works best for each type of coffee bean to bring out its best qualities. This is accomplished through both experience and personal preference.
Failure to roast the beans properly will result in either charred beans (which taste bitter or burnt) or underdeveloped beans (which taste like... nothing). Either way, you can ruin a perfectly good bean with careless roasting. Sad.
Of course, perfectly roasted beans still need to be brewed properly too!