Contains the full state of the quantum system.

This is in contrast to classical mechanics where e.g. the state of mechanical system is given by two real functions: position and speed.

The wave equation in position representation on the other hand encodes speed in "how fast does the complex phase spin around", and direction in "does it spin clockwise or counterclockwise", as described well at: Video "Visualization of Quantum Physics (Quantum Mechanics) by udiprod (2017)". Then once you understand that, it is more compact to just view those graphs with the phase color coded as in Video "Simulation of the time-dependent Schrodinger equation (JavaScript Animation) by Coding Physics (2019)".

Relates particle momentum and its wavelength, or equivalently, energy and frequency.

The wavelength relation is:
but since:
the wavelength relation implies:

$λ=ph $

$v=λfE=pv$

$fv =ph f=hvp =hE $

Particle wavelength can be for example measured very directly on a double-slit experiment.

So if we take for example electrons of different speeds, we should be able to see the diffraction pattern change accordingly.