For security reasons, the site keeps track of every login. This is always available to the site administrator. For users, the optional You last logged in... message on the sidebar reminds you of your last login. The View your login history link shows your own login history. The information can be helpful to detect spurious account activity. It also reminds users that they are being monitored, which discourages bad behavior.
hmmm ... let's discus ...
This sidebar feature is now disabled. While this can be useful for security, it is only useful immediately after logging in. But the block shows the information forever, page after page. This is just annoying and a waste of space.