Linked from
The 16 pages that link to Bloch's theorem, each with the reason it gives.
Band gapRelated: Periodic crystal potentials produce the energy bands whose edges define the gap.
Bravais latticeRelated: It uses lattice translations to constrain electron states in crystals.
Brillouin zoneRelated: It allows crystal states to be indexed by wave vectors within one zone.
Fermi surfaceRelated: It makes crystal momentum a useful label for electronic states in a solid.