What is the use of WKB after Schrodinger equation is established?

[wdlang]

i think every system is accurately described by Schrödinger equation.

so what is the point of using old quantum mechanics methods?

with Schrödinger equation, at least numerically, you can solve the eigenvalues and eigenvectors readily and accurately. So what is the point of using approximate methods like WKB or even some strange quantization methods such as EBK?

 

[DrDu]

There are several reasons:
a) Often, the numerical solution of the SE provides little insight into the phenomena you want to describe.
b) A numerical solution of the SE rapidly becomes prohibitive for complex systems.
c) There are fields like quantum chaos which rely heavily on semiclassical approximations to define chaos in the quantum regime.