The other informs you, as admin of the forum, when there is more than one account being accessed from the same computer. While sometimes it is legit (family members, friends accessing from the same computer) a lot of times you catch troublemakers signing up with multiple accounts to cause trouble. This helps towards catching them so you can ban them.
I would recommend visiting www.vbulletin.org and browsing their hacks/products add-ons, well worth the time you spend. A lot of the answers you seek with your question is dependant on what you want out of your forum, etc. - a lot of times it's all personal preferences.