John gave the book to Steve / John le dio el libro a Steve
John gave the book to them / John les dio el libro a ellos.
John gave the book to Steve and Al / John le dio el libro a Steve y Al.
1) is there a simple explanation for why the ind obj pronoun must be included in the 1st example?
2) why is the ind obj pronoun in the 3rd example “le” instead of “les”?