The tab bar helps you quickly switch between the files you're working on. When you select a file in the file manager, the text of the file shows in the editor, but this doesn't create a tab for the file until you focus the editor. This lets you browse files without thinking about tabs.
There are a few useful shortcuts related to tabs:
- ⌘w – close the current tab
- ⌥⌘t – close all tabs except the current one
- ⇧⌃← and ⇧⌃→ – move the current tab to the left/right in the tab bar