The Google search bar saves you the step of opening a new window or tab to search for something, should you endeavor to copy and paste or type a search term into it. With Search Highlighter, you save another step between you and your Google results, needing only to highlight a word in Chrome and hit return.
The extension adds a menu bar below your bookmarks bar with either a Google or Bing search bar (your choice). Any word you highlight shows up in the search bar; hit return or click the magnifying glass button to initiate a search.
If you click the settings button to the right of the search bar, you can set the extension to open a new tab for your searches. Problem is, this setting only adds another magnifying glass (with a plus mark); hitting return still punts you from your current page to the search results page.
In settings, you can also move the Search Highlighter to the bottom of Chrome, but that's a bad spot because the search bar is frequently obscured by the status bar when loading a page. You can also set it to display between 1-99 terms in your search history.
Lastly, I experienced one odd behavior when writing this post in Google Docs on a MacBook Pro. The cursor in Google Docs is two lines off from my mouse arrow -- no matter where I click in a Google document, the cursor shows up directly two lines below. This might just be something with my Mac, but it's never happened until today after I installed the app. And when I disabled the app, each time my cursor accuracy returned in Google Docs. Probably a deal-breaker for me, but your mileage may vary. The developer states the extension is in beta, so perhaps there are still a few bugs to work out.