Most importantly it highlights (optional) the search words in the document. Therefore, you don't have to CTRL+F and type your search world again after you visit one of the search result which is 10 pages of text and look for the relevant section.
Second, the pop-blocker of the toolbar makes a "click" sound and cursor changes momentarily to let you know that a pop-up has been blocked. With opera, you have no idea this happened. So when the pop-up window has some actual content, with goolgle toolbar you can allow the pop-up to see the content. In opera, you wouldn't know what happened.
And finally, the toolbar has a translate button which uses the google translator for converting webpages into english (and other languages) with one click. With opera, you have to manually give the url to google (or altavista) for translation.
See toolbar.google.com for complete details of the toolbar. It does a lot more like autofill, etc.