Linked from
The 31 pages that link to Complete graph, each with the reason it gives.
Graph minorRelated: Complete graphs often serve as canonical excluded minors.
Brooks' theoremRelated: Complete graphs are one of the two exceptions to the degree bound.
Graph homomorphismRelated: Maps to complete graphs encode proper colorings of the source.
Turán's theoremRelated: The forbidden complete subgraph is a clique of the specified size.
Burr–Erdős conjectureRelated: Graph Ramsey numbers ask how large a complete host graph must be.