Postfix is a Mail Transfer Agent, so broadly it can accept mail to send to other people, and it can deliver mail locally.
Installing Postfix on a desktop/laptop/netbook end-user machine is something people normally do when they want to be able to queue up mail on the local machine when it's disconnected, for later transmission to another server. For instance, if you want cron scripts to be able to send mail off your desktop to your "real" address, this would be a good reason to install Postfix. (And this is the reason it's pulled in by things like mdadm and devscripts.) You can find descriptions of how for instance to get Postfix to relay your mail through Gmail. This is case #1 in your list.
Secondarily Postfix is useful when you want to deliver mail to local mailboxes. It is fairly rare these days to do this onto your desktop, but certainly still valid.
In case #2 Evolution will take care of spooling and transmitting messages itself.
In case #3 with Gmail, normally you'll use its web interface. If you want to use a mail client like say mutt with it, or you want to be able to send mail from the command line to gmail, postfix would be a good addition.