What it can be used for/how useful is the info aside…
Your Discourse site stores a user’s registration IP address for as long as the user exists on the site. It also stores a user’s ‘last IP address’, which is updated each time a user logs into the site. These IP addresses are removed from the site when a user is deleted or anonymized, though can be retained in the logs if the log anonymizer details site setting is left enabled.
IPs are also stored in the topic_views table, but only for anonymous users. That table also only records when a user/ip first views a topic, and not any subsequent ones.