Plane waves in quantum mechanics are usually the eigenstates of the momentum operator, which is what makes them very useful. Momentum conservation is the manifestation of the translational variance in space, which is arguably what makes plane waves also very useful in classical contexts, whenever one deals with a homogeneous media.

Mathematically, plane waves correspond to the Fourier expansion, which is also a very convenient mathematical tool.

On a more general level: expanding in terms of the appropriate orthogonal basis is often a good idea.

Plane waves in quantum mechanics are usually the eigenstates of the momentum operator, which is what makes them very useful. Momentum conservation is the manifestation of the translational invariance in space, which is arguably what makes plane waves also very useful in classical contexts, whenever one deals with a homogeneous media.

Mathematically, plane waves correspond to the Fourier expansion, which is also a very convenient mathematical tool.

On a more general level: expanding in terms of the appropriate orthogonal basis is often a good idea.

Plane waves in quantum mechanics are usually the eigenstates of momentum operator, which is what makes them very useful. Momentum conservaion is the manifestation of the translational variance in space, which is arguably what makes plane waves also very useful in classical contexts, whenever one deals with a homogeneous media.

Mathematically, plane waves correspond to Fourier expansion, which is also a very convenient mathematical tool.

On a more general level: expanding in terms of the appropriate orthogonal basis is often a good idea.

Plane waves in quantum mechanics are usually the eigenstates of the momentum operator, which is what makes them very useful. Momentum conservation is the manifestation of the translational variance in space, which is arguably what makes plane waves also very useful in classical contexts, whenever one deals with a homogeneous media.

Mathematically, plane waves correspond to the Fourier expansion, which is also a very convenient mathematical tool.

On a more general level: expanding in terms of the appropriate orthogonal basis is often a good idea.