Drupal has some pretty big nested data structures, so this feature is a nice way to see those values. Click on the Locals tabpage.
Arrays can be expanded/collapsed by clicking on the triangles. This makes it alot easier to navigate the local variables.
Also these variables can be changed by clicking on a variable, then clicking on the pencil icon, or by pulling up the context menu, by either ctrl-clicking or click and hold on a variable.