When sprinters train, they may affix weights to their ankles to build running strength. The idea is that if they can run fast with the weights on, they will run even faster in the actual competition with the weights off. This offers a great technique in career performance. By taking on added side work, such as serving on a non-profit board, or working on weekend projects, you can build much needed additional capacity that can be quickly shifted to your day-job as needed.
Let's say an opportunity for advancement suddenly presents itself, and it requires that you draft a new strategy and operational plan for your organization in a short amount of time. It turns out you had done something similar for your non-profit board work, and you can adapt some of the same templates for work. Furthermore, you can cancel a couple of weekend board meetings (taking your training weights off) to blitz through this. It's like having a flywheel with stored energy that you can tap as needed.
The beauty of doing non-binding external work is that it allows you to train and grow your skills but not be burdened with extra responsibility. It's not as good an idea to train new skills by volunteering for a greater workload at work - that can have the reverse effect of weighing you down with low-value tasks that you cannot quickly shed. It also sets you up with the risk of a public and debilitative failure by promising to deliver on things you aren't already experienced at. It's one thing to miss on your home garage project and trash months of your own work, quite another to do that at your workplace.