It lets you set a single “seed color” and from that it makes you a complete color scheme. I think “anyone can be a designer” features like these could be really valuable for a forum, as it makes it very easy for anyone to style their forum to their liking - in accordance with style standards determined by Discourse - which in turn reduces the “every Discourse forum looks the same” effect.
The color theming system is a great first step, but your average forum admin will not have the design sense (or patience) to make a good looking and readable forum design.