Interesting question. I'm kind of surprised no-one has bitten yet, so I'll kick off.
These questions are not that easy to answer in a single sentence.
What is the purpose of your photography?
Nowadays, the purpose of my photography is to earn a living. At least, most of what I shoot is. I feel rather privileged of being able to say that I've turned my hobby into work, and I'm enjoying every minute of it. I still shoot for fun, and not all clicks are intended to make money with. I still do what I used to when I started out in photography, which is to experiment with new techniques and such. The purpose of that is to stimulate creativity, innovate, learn and improve.
How it helps me or my family?
I feel the approach of experimenting works fine, for me. I feel I still have a lot to learn, yet am consistently improving in image quality, both technically as well as aesthetically.
The money earned most definitely helps to feed my kids, although I presume you refer to "helping" in a more philosophical meaning. One thing has changed. I seldom take photos of my kids or family gatherings anymore, as I seem to be unable to do anything with them. (Gave my wife a decent DSLR to take over in that department:))
How is civilisation in large benefiting from photography?
Generally, I think civilisation is benefiting from the stories told through images by photojournalists. If handled properly, photojournalists capture events as they occur, without prejudice. Key element here is "If handled properly", I think, and I'm fairly convinced that isn't always the case. I know how much trickery can be preformed with photographic data. (a good example: http://photoshopdisasters.blogspot.com/2008/07/iranian-govt-persian-pixels-pwned.html - no political intent or opinion intended).
Civilisation in large is also benefiting from the technological developments - the innovations in hardware as well as software are being applied in other fields, such as (but not limited to) for example medical imaging techniques.
One thing I'm more certain about is that civilisation is not benefitting from my photography. My product only assist in selling the products of my clients. Fine with me, but nothing more than that.
I can't comment on Peter Vander Meulen's words, as I'm unfamilliar with that discussion. But there are plenty of databases online, some of them exceeding a catalogue size of 10M, documenting that aspect of the "sign of the times". Fashion changes, cities change, the faces change, heck, even countries change, and new photos are added on a daily basis by the thousands to pinpoint those changes. Personally, I have no desire to maintain a personal collection of those changes or events.
So far, my 2 cents worth,