Most recently, I volunteered to improve a design - not by stating how it was incorrect or risky (testing) but by coming up with several other suggestions. I ended up owning the design of the feature and the driving the discussions to conclusions and agreement.
On numerous occasions, I've volunteered to deal with something everyone seems to be avoiding that needs doing. I've volunteered to speak out with a personal risk when I know something needs to be said that others would want to say but don't. Volunteering is my way of contributing. I'm very seldomly assigned something to do, instead I'm actively looking at what needs doing and what I should volunteer for. I don't need to be a manager to be an active player in the direction I'm heading and we're working towards together in the team.
I realized volunteering comes naturally for me. I volunteer for boards of non-profits that I believe in. I volunteer to run a kids computer club for 1st graders to teach them about versatile, collaborative creating with computers instead of programming. I volunteer to organize meetups to meet great people that give me extra energy with their ideas and enthusiasm.
While at university, I volunteered for many student organizing activities. I held several financial responsibility positions organizing events with large audiences. So many of those lessons have been valuable, defining moments on what I do now.
Going all the way back to when I remember volunteering for the first time: my sisters needed me. I volunteered to teach her Swedish so that she would pass a class without meeting a teacher she did not get along with. She aced the exam and was barely allowed on the next class level. I felt useful.
There's so many things that would not have happened without volunteering. It's rewarding and it drives me forward allowing for all options. I suggest everyone should try volunteering. I've been surprised on how much of a difference that makes.