Linked from
The 12 pages that link to Dirichlet boundary condition, each with the reason it gives.
Boundary conditionsBroader topic: It is the direct specification of solution values.
Neumann boundary conditionCompared with: It fixes boundary values rather than normal derivatives.
Helmholtz equationRelated: Imposing it yields one common family of Helmholtz eigenvalue problems.
D-braneRelated: The name D-brane comes from this condition on open-string endpoints.
BraneRelated: Open-string endpoints obey this condition in directions transverse to a D-brane.
Friedrichs' inequalityRelated: Zero Dirichlet data exclude constant functions other than zero.