Agile release planning is where the big ticket wins of Agile come from. It's what leads to improved throughput, increased value, better time to market, and the ability to respond to change. Sure, the other benefits of doing Agile well are nice--improved morale, lower defects, and so forth--but you can get those using other methods. Agile release planning is what sets Agile methods apart from other approaches. It's the reward you get for using Agile.
Most companies I work with don't reap that reward. Some (many!) focus entirely on iteration planning and stand-up meetings--Sprints and Scrums--which turns Agile into a glorified project management technique. (Yuck.) Of the remainder, only a few put in place the necessary engineering practices that allow Agile release planning to succeed.
But for those that do, the rewards are glorious. You ship faster, learn more, take advantage of opportunities and--no kidding--create brand new opportunities through rapid experimentation and refinement.
Agile release planning is the reward you get for using Agile. It's a shame so few "Agile" companies take advantage of it. Read about it here.
I'm in Sweden next week, speaking at the Øredev conference, so I won't be posting a new practice. When I come back the following week, I'll post one of the remaining practices from the Developing chapter. Refactoring is the next on the list, but let me know if you'd like something different.