If IE was open source, anyone could add features to it for instance.
Believe it or not, a very large amount of the code of Google Chrome was not written by Google. In fact, you can find substantial amounts of code from Apple and Nokia in Google Chrome. And some from the KDE project as well. Welcome to open source.
Google is itself an open source success story. If there was no open source, they might not even exist. Their whole business is built on Linux, Python, Java, etc. and they customize everything for their purposes.
If you want to know more about this software development paradigm, this is a good book to read: