I do a lot of pro bono work for non-profits. Mostly, it's because supporting organizations doing good work is the right thing to do. And while money is always important, capacity-building is also a valuable commodity.
Nine years ago, I delivered my Power of Your Ideas workshop to a Leadership Triangle cohort. Someone in that class called me yesterday to hire me for a talent development training. She'd found my 2013 session useful and held onto the materials for all these years. When she saw the need for communications training for her team, she knew who to call.
The moral isn't that you should use pro bono work as biz dev or sales. (Seriously, do you want an almost-decade-long sales cycle? You. do. not.) The moral is that being of service puts good out there in ways that encourage it to come back -- in ways you may not expect.