In the answer of my question:

On the full reducibility of representations of reductive Lie algebras

James E. Humphreys replied to me saying that:"the notion of "reductive" for a Lie algebra in characteristic 0 has no intrinsic interest, unless you study the Lie algebra of a Lie (or algebraic) group and relate their representations carefully."

Can please someone explain that to me or give to me any reference? thank you!