What does the mitzvah of "walking in His ways" (v. 9) teaches us that we do not already know from the requirements to, "love your fellow as (you love) yourself" (Vayikra 19:18)?

Rambam explains that the key focus here is that, "we are commanded to emulate the Almighty as much as possible." I.e. when we act in a manner of graciousness, compassion or benevolence, it should not be merely out of ethical imperative, or for the general benefit of society, but rather, in an attempt to resemble God - "Just as God is referred to as being ... likewise you are to be..."

Practically speaking, this will ensure that a person will not allow himself to become emotionally "carried away" to an extreme. For in attempting to emulate God, one will always be conscious of the need to balance one's attributes - like God, whose qualities of kindness and severity are in perfect harmony.

(Based on Likutei Sichos vol. 34 p. 153ff)