I explained bits of it in posts below, but the quick answer is that the login system ensures that you know who you're talking to.

Posting the IPs was definitely a two-edged sword. It was not only the use of identities that could have a dark side, IP addresses exposed personal information such as where a person lives. You can pin down the numeric address to a Zip code with 95% accuracy in the US, less accuracy elsewhere. And an IP address can be correlated with a domain name, exposing things such as one's employer, or at least ISP.

Even cooler, or worse depending on your viewpoint, sometimes you can match an IP to a specific computer inside an organization. Years ago, when I looked at the logs for the pic viewer for the Mars rover images, I saw a lot of visitors whose IP address resolved to msss.com, Malin Space Science Systems, mission operator and principal scientist. And the really cool thing is that their individual IP addresses, when converted to computer names, turned out to all be references to characters in Kim Stanley Robinson's Mars trilogy. They maintained a poker face in public for fear of encouraging anomalists, but deep inside, they're dreamers just like the rest of us.

Anyway, I probably never again will expose IP addresses, to protect privacy. But most systems collect them, and they'll always be there in the database if we have a problem involving identity.