Hmmm, it's the search button. Basically searches the device for anything related to what you type. Or if you hold it down you speak what you want to search for. Searches your apps, contacts, bookmarks, all kinds of things. Will even search the Internet if you don't have a match on your device.
I usually use it to quickly search for something on the Internet using the voice input.
Try it, you might like it.
Thanks, I was agreeing with the OP until i read yours, did not know about the hold search function to bring up voice search, was actually wanting that functionality. Glad it uses google too, IDK why, many times when i used search it took me to Wikipedia instead of using the internet to search.