There's a subtle comment in this chapter on which I would like to focus. One of the reasons the "other side" exists is so that Tzadikim can be rewarded for their actions.
You would think that the reward for good behavior comes from the "good" side, but surprisingly it doesn't. It comes from the "other" side. There's a number of reasons to explain this, and frankly some of them are well beyond my own ability to understand, but this actually explains some otherwise difficult statements by Chazal.
In Pirkei Avot we're taught to not serve HaShem in exchange for a reward. If we look at the Komarna Rebbe's peirush, Notzer Hesed, he highlights the original phrasing of the Tanna as, "Serve in order not to receive a reward." In light of the Baal HaTanya's comment in this chapter, we can make some sense out of the value of serving HaShem in order to receive no reward.
As we know from elsewhere in the Tanya the Tzaddik utterly hates evil. Since reward shares a common source with evil, the Tzaddik is repulsed by reward. From here we can see how the Tzaddik would do whatever necessary to serve HaShem and receive no reward in exchange.
Perhaps this even provides the opening and motive for Mosheh Rabbeinu to have sinned on purpose at mei merivah. Some interesting things to consider from a seemingly offhand comment in the Tanya. Clearly I haven't begun to reach the inner depths of the wisdom of the Tanya.