  Vector is a physical quantity and it does not depend on any co-ordinate
system. It need to be expanded in some basis for practical calculation and its
components do depend on the chosen basis. The expansion in orthonormal basis is
mathematically simple. But in many physical situations we have to choose an
non-orthogonal basis (or oblique co-ordinate system). But the expansion of a
vector in non-orthogonal basis is not convenient to work with. With the notion
of contravariant and covariant components of a vector, we make non-orthogonal
basis to behave like orthonormal basis. The same notion appears in quantum
mechanics as Ket and Bra vectors and we compare the two equivalent situation
via the completeness relation. This notion appears in the differential geometry
of a metric manifold for tangent vectors at a point, where it takes into
account the non-orthogonality of basis as well as non-Euclidean geometry.
