Actually I just got a job as a photographer with a studio here in town... though they only do portraits and things of that nature, no party pictures or snapshots... still, I've learned more in one or two training sessions (and I still have a few more weeks of training to go) then I have on my own reading books and articles about photography... (I've got my 'party' camera and a pseudo-DSLR... Minolta DiMage 7i... though I'm saving up a few weeks to get the Olympus E-300 DSLR).
As for the "purpose" of taking pictures, there is none. Most nights I go out I don't bring a camera along. The only times I'll have one with me is if it's a 'special occasion' such as a friends birthday, a christmas party, halloween, etc etc etc. When I wasn't used to going out much, several years back, I'd bring a cam everywhere - now that it's become commonplace, it takes something worthwhile to get me to bring one out with me. If you wanted to look deeper, I guess the "reason" I take pictures at parties is so that I can look back at them when I'm a senile old 89 year old and say "Da*n, I've had a good life".
I don't get the question you asked me at the end. Could you rephrase it? From what I understand, you and your co-workers will pay me to take pictures of the city I live in?