Linked from
The 14 pages that link to Inner product space, each with the reason it gives.
Fourier analysisNarrower topic: Fourier coefficients arise from projections in spaces of functions.
OrthogonalityNarrower topic: Orthogonality is defined by the inner product that equips this space.
Orthogonal projectionNarrower topic: Orthogonality and distance depend on this structure.
Spectral theoremNarrower topic: Orthonormal eigenvectors are defined using its geometry.
Geometric algebraRelated: Its metric information determines how vectors multiply geometrically.