Linked from
The 6 pages that link to Adjoint representation, each with the reason it gives.
Lie groupRelated: It describes how group symmetries transform infinitesimal directions.
Lie algebraRelated: It shows how symmetry transformations act on infinitesimal generators.
Jacobi identityBroader topic: The identity is equivalent to the adjoint action preserving brackets.
Ado's theoremRelated: It gives a natural first action, though its kernel is the center.