Linked from
The 17 pages that link to Orthonormal basis, each with the reason it gives.
Rotation matrixRelated: The columns of a rotation matrix form an orthonormal basis.
Orthogonal projectionRelated: Coordinates along these vectors make projection especially simple.
Probability amplitudeRelated: Amplitudes are basis-dependent coefficients of a state vector.
Kronecker deltaRelated: Its basis-vector inner products are encoded by delta values.