"more relevant search results"
For example, we can make search more relevant and interesting for you by including photos, posts and more from you and your friends. When signed in with Google+, you’ll find tailored results and profiles of people you know or follow, and people who know or follow you may see your posts and profile in their results. You can change your profile settings at any time if you don't want Google and other search engines to index your profile. Learn more.
When you're signed into your Google Account and have Web History enabled, you can get more relevant search results based on your Web History. Web History includes your searches and other web activity. Your search results also may be customised using search activity from your computer even if you're signed out. Learn more.