No, it's not compulsory, don't worry. It's one level above the level normally reached in high school, and three levels above what is minimally required to graduate, at least in New York State. In my school they choose who will eventually take Calculus by age ten and "track" your math sequence accordingly, because it involves essentially skipping a year. It's fairly common though and not a big deal.
As Turlough said, Calculus basically helps you get a better understanding of mathematical functions. One basic and fundamental facet of calculus is the derivative, which measures the instantaneous rate of change of a function. This is also useful in physics and other sciences, since functions are often used to model phenomena.
And for me it's over (until next year that is)!