Linked from
The 14 pages that link to Simple graph, each with the reason it gives.
Directed graphRelated: Simplicity constrains loops and repeated edges independently of direction.
Line graphRelated: For simple inputs, the line graph is also simple.
Fáry's theoremRelated: The theorem's standard statement assumes this restriction on the graph.
Conway's thrackle conjectureRelated: The standard conjecture is framed for simple graphs.