Does the departure tax basis reset reduce capital gains tax if I sell my investments later as a non-resident?

For property that was actually subject to the deemed disposition, yes, being deemed to sell and immediately reacquire it at fair market value on your departure date effectively resets your cost for that property going forward, so any future gain measured from that new, stepped-up value reflects only growth that happens after you left, not growth that occurred while you were still a Canadian resident and already taxed once through the departure tax itself.

This reset matters most in situations where you eventually become a Canadian resident again and Canada resumes taxing your worldwide income, since it prevents Canada from taxing the same pre-departure growth twice. It's less relevant, though, for property that was never subject to the deemed disposition in the first place, Canadian real estate is the clearest example, since it wasn't taxed on the way out, there's no need for a reset, and its original cost simply carries forward unaffected by your period of non-residency.

Because this only helps with property that actually went through the deemed disposition, and doesn't apply to excluded categories, knowing which of your assets fall into which bucket is important for understanding what your basis actually looks like if and when you sell later, whether as a non-resident or after returning.