Whilst volunteering doesn't pay a salary, there are many other benefits.
for those who have been out of the job market for some time, voluntary work may be 'just the job'. You have control over the hours you work, where you work and what you do.
people who are retired from the work place (for a variety of different reasons) have a wealth of experience at their disposal, people want to remain active and involved, voluntary work is an ideal alternative to the great divide between 'working' and 'not working'.
maybe you are a photographer, or enjoy gardening, maybe you like sport or like to cook, you might possess creative skills you think others may enjoy. There are many organisations that provide services to people who would thoroughly enjoy your input, and give you the opportunity to share your enthusiasm for your subject.
An improved 'sense of belonging'
we all like to feel a part of something, the work place is where we most often meet friends, however if you don't work you may not have the opprtunity to meet new people. As a volunteer you will work with others toward a common goal. If you feel isolated at home or want to widen your social circle volunteering may be the route that would most suit your purpose.
some people are happy to go to work and earn a salary, they might be reasonably comfortable with their job, but feel they want something a little more challenging (possibly a new career). If you want to try something new but can't risk giving up your job, voluntary work may well provide a risk free transition. Most voluntary organisations provide their volunteers with professional (and often certificated) training
If you haven't been in the job market for a while, how will you prove your skills and abilities, who will confirm your strengths? Being involved in voluntary work says something about who you are, and provides you with a referee who has experience of your work ethos.