Daniel notes that self-delusion is a necessary component of all voluntary work.
Any human activity can be over-analysed into oblivion, especially if the terms of reference for "meaning" are impacted by a view of the real world that is overly abstract. Even paid work needs some self-delusion because no matter how much money you earn, the old maxims bite back - "you can't take it with you" and "no point earning it unless you can enjoy it".
Before I started with voluntary software work, I was heavily involved in voluntary charity work and the motivation is much the same. "The grand scheme of things" will never be sufficiently affected by any activity such as to provide the sole justification for that activity. The vast majority of people and the vast majority of actions simply are not and cannot become that important. (This is where religion is important for some people, the hope that despite all evidence to the contrary, acts of random kindness have some meaning to the person doing the act instead of just to the recipient.)
Voluntary work is generally for the benefit of others. In free software it is about writing code to solve our own issues in a way that others can use without facing the problems that we had to solve. Writing new code to solve a problem or fixing bugs in existing code to solve a new problem can only help others if the solution is shared. So the validation of that work is in helping someone else accomplish a task without having to spend the time fixing that issue, leaving them time to fix the next one.
Working on free software of all kinds is worthwhile simply because it is a way of helping others.
"When all things are considered", most people working on Debian are the lucky ones. Born into an environment where good education is available and where parents/relatives were able to indulge our fascination with electronic and computerised widgets that had no direct bearing on whether our basic human needs were met with regard to food, warmth and health. Trained at great expense and able to use that training not only to meet our personal financial needs but also to make time available for other unpaid tasks that benefit others who do not have those things. Living in a time of peace and stability, where the more esoteric and "frivolous" tasks can be undertaken without causing hardship or neglect of others. (My nephew frequently sums up this whole idea as like having won the lottery just by existing.)
There is an element of justification that can be derived from such a view - an element of responsibility and duty. After experiencing all those good things and after realising that we have such an opportunity, it can engender a feeling of obligation to do something that justifies what we have already received to balance out those who have the skills to do what we do but, for reasons of birth alone, do not have that opportunity or the means.
That is a form of secular humanism - validation of our own situation in the use of our skills and time to make things easier for those who are less fortunate.
OK, Debian isn't about providing clean water for all, or the redistribution of food and medicines to those without but the organisations that do that work can be assisted indirectly by making the systems used by such organisations work more efficiently, with greater stability and with more flexibility.
Not everyone can (or should) be on the front line of whatever acts of kindness will solve the problem of inequality but there is an obligation on those of us who have received the benefits of inequality to do what we can to make it easier to redraw the balance.
Debian is one such way and it is a truly international way of benefiting others too. It is more beneficial than the charity work that I did previously - chiefly because that was a very local service that was very labour-intensive and so could only benefit small numbers of people.
Improving Debian won't make us more important to the universe, life and everything (the answer to that is 42 as we all know), but it will help others do what they do, it will improve their lives and their ability to spend their time on what they want to do instead of having to fix problems that we have already solved.
Debian, to me, is about sharing solutions. Helping someone else do something that Debian could not do before, without requiring them to fix all the problems that I had to solve to devise the solution in the first place. Sharing not just the solution but the method of the solution and the means to extend the solution to new problems.
We, as Debian developers, have been granted the access and tools to solve these problems for the benefit of those who do not have upload privileges. Those rights come with responsibilities (as always) to use our skills and time to find the solution and without making things more difficult in the process.
In the grand scheme of what I can actually do as opposed to what might be possible if X, Y and Z were different, what I do in Debian and Emdebian is worthwhile. In fact, it is just as worthwhile as my paid non-IT employment in healthcare provision because, within healthcare, I can only help the single person in front of me. In Debian, I can help all those people who I will probably never meet but who want Debian to do something that I am able to achieve.
What is particularly nice about Emdebian is that there are people who tell us that our work is worthwhile by putting up the hard cash to fund work sessions and conferences that bring the various developers into one place. The international aspects of Debian can be a barrier in some ways and things always happen more quickly when the various members of the team can be brought from the four corners of the planet into one place for a week or so. It is these gatherings that boost the motivation and the enjoyment of making progress towards a common goal. Debian can be a solitary world where communication is limited to plain text. Probably the best way to "oil the wheels" in Debian is to get a group of Debian people together and just let them hack.
Voluntary work of all kinds requires personal motivation and working alone with only limited means of communication with others can be quite demotivating. Getting people together is a very useful antidote.