After 673, we learned the Sage's powers were divided between Senjutsu and the Rinnegan. Which in my mind is some genius planning. Think about it we learned of the Sage when Jiraiya revealed Sage Mode, and Pain revealed the Rinnegan. Personally Sage Mode was always felt a little out of out of sync with the rest of the series. It used different energies never heard of, and Naruto already had the Fox's powers, he didn't need another super mode. As Naruto continued to learn of his jinchūriki powers, I thought Sage Mode would be forgotten like Goku's Kaio Ken. But Sage Mode continued to be useful and prominent throughout the series. After 673 everything clicked, and shred some light on previous arcs.
The biggest was the Pain Arc, that arc despite having some very awesome fights; was also a philosophical battle between Nagato and Naruto. Analyzing the arc, we can see the arc was a struggle over who should be the Sage's successor. With Naruto and Nagato playing as Ashura and Indra respectively by their philosophies and their powers. Naruto having the body of the Sage, Sage Mode and the belief that people can truly understand one another like Ashura believing love was the answer for peace. Pain with the Rinnegan and his belief that a superweapon of the Tailed Beasts will be the answer is similar to Indra's belief that power was the answer.
The parallel continues in the War arc before Naruto and Sasuke join the final battle, they each have an incredible battle that nearly ended in their lost. But the interesting idea is the opponent that they nearly lost to had part of the sage's power as well. Naruto nearly lost to a fully healed Nagato, who as stated before had the Rinnegan. While Sasuke nearly lost to Kabuto, who had Sage Mode like Naruto, having the body of the Sage. It continued when Hashirama had Sage Mode, which was fitting as he was Ashura's incarnation as well and so he had the body of the Sage like Ashura . If I'm wrong tell me how and if I missed anything.