If a visitor is not logged in, Moodle displays small Login links in the upper-right corner and bottom-center of the page. However, the links are not very noticeable. The Logged in user block is much more prominent. One advantage to the Logged in user block over the small Login links is the block's greater visibility.

If you want the Login block to be displayed on every page of your site, set Page context to Display throughout the entire site and ensure that the locks are unlocked and the eye is open:

After the user logs in, this block displays information about the current user. The information is taken from the user's profile. It also displays a Logout link.