It is used to make precise statements from ideas that automorphic forms are in a sense functorial in the group G, when k is a global field.
From a reductive algebraic group over a separably closed field K we can construct its root datum (X*, Δ,X*, Δv), where X* is the lattice of characters of a maximal torus, X* the dual lattice (given by the 1-parameter subgroups), Δ the roots, and Δv the coroots.
A connected reductive algebraic group over K is uniquely determined (up to isomorphism) by its root datum.
A root datum contains slightly more information than the Dynkin diagram, because it also determines the center of the group.
The full L-group LG is the semidirect product of the connected component with the Galois group.