When a variable/function/identifier is selected, it helps to know where else it is being used.
In this image, variable
file is selected in line
other locations where the same variable is used, also gets highlighted.
When the references are spread across the file, a quick overview can also be found. As in this image, the occurrences are shown in the right sidebar. (See all the light blue markers on the right side of editor view.)
Some times this feature is annoying. Its easy to disable/toggle it quickly.