Many code editors have it.
A double-click on a word (a variable name, a table name etc) and it gets highlighted in every place where it is mentioned in the text.
The screenshot is from Notepad++
This feature is mainly used when I need to know whether the word that I am focused on at the moment is used somewhere else near in the code.
Of course there is a workaround - F3 (Find Next), but having highlighting would be nice.