When you install BuddyPress it adds the BuddyPress features to the standard WordPress toolbar.  This allows logged in users to access their BuddyPress features from the toolbar at the top of every site and dashboard on your network.

The toolbar is used by members to access:

1.  Their WordPress blogging tools

2.  Their BuddyPress social features

When you hover your mouse over Howdy, User, this expands to link you to your Profile Screen as well as a Log Out link and your BuddyPress social features.