The Fourier and Laplace transforms are defined by testing the given function f by special functions (characters in the case of Fourier, exponentials in the case of Laplace).

These special functions happen to be *eigenfunctions* of translation: if one translates a character or an exponential, one gets a scalar multiple of that character or an exponential.

As a consequence, the Fourier or Laplace transforms *diagonalise* the translation operation (formally, at least).

Whenever two linear operations commute, they are simultaneously diagonalisable (in principle, at least). As such, one expects the Fourier or Laplace transforms to also diagonalise other linear, translation-invariant operations.

Differentiation and integration are linear, translation-invariant operations. This is why they are diagonalised by the Fourier and Laplace transforms.

Diagonalisation is an extremely useful tool; it reduces the non-abelian world of operators and matrices to the abelian world of scalars.