Okay I have been engaged for exactly a month and a day

Here are my questions...
-What is the point of getting engagement photos done? Do you send them out with invitations? Are they important? I want to get some done, just because my fiance and I have never had any pictures done, but I don't want to spend a lot of money. What's a reasonable amount for e sessions?
-Are engagement parties necessary? How soon after being engaged should they happen? Can they be informal (as in sending out invites on Facebook?) Is it okay for it to just be a small get together with a few friends?
I'm trying to budget wisely, because I want to spend most of the money on the actual day! Thanks so much for all of your help and advice!