我念书的时候就没明白 linear algebra 的 真正用途

For example,

learned how to cacluate eigenvalue and eigen vector, but never knew (the chinese version textbook and the profesoor) the importance of the real applications. 

Because my major is CS, I never got chance to understand "decompose the matrix" by eigenvalues and eigenvectors, though this is the basic concept and application in control system. Not to mention other more advanced applications