The Google toolbar is a downloadable browser toolbar for the Internet Explorer and Firefox browsers. It allows users to perform many functions of the Google search engine without actually visiting the Google site. It is proprietary freeware developed by Google; however, it is frequently offered as a secondary download bundled together with other software.
To a certain extent, the Google Toolbar is obsolete. It was created in a time when browsers didn't have built-in search. Pretty much any modern browser, including Chrome, allows direct searching on Google or other search engines without an add-on.
Techopedia explains Google Toolbar
The Google toolbar was first introduced in 2000. It was only available on Windows 95, 98, 2000 and NT, and version 5.0 of Internet Explorer. The features offered were direct access to Google search on any web page, Google's PageRank of a particular page, and automatic selection and highlighting of a search term within a page. In 2003, version 2.0 of the toolbar was released. It offered additional functionality, such as a pop-up blocker and the auto-fill feature for automatically filling in data on various web forms. In September 2005, Google released a version of the toolbar for the Firefox browser, making it compatible with the Mac and Linux operating systems in addition to Windows.
One reason for its continued popularity is the ability to get the Google PageRank for the current site. Many in SEO have the Google Toolbar for this purpose even though they don't need the other functionality.