This is the most important course in highschool Mathematics because it connects the student's knowledge of Algebra and Geometry to start learning Calculus. It ends at Limits and Continuity which are building blocks for understanding the need for Calculus. Remember that it is very important to keep practicing Pre-Algebra, Algebra-I and Agebra-II concepts continuously to do good in Precalculus.
Precalculus is an important pre-requisite for Calculus. This course dives into new concepts of Derivatives and Integrals based on Limits. A good understanding of discontinuities, limits and functions in general is very good to have before delving into Calculus. A good sense of graphical representations of different functions is a handy tool at this point in Mathematics.