Excerpt from Google Drive's Terms of service:
"When you upload or otherwise submit content to our Services, you give Google (and those we work with) a worldwide license to use, host, store, reproduce, modify, create derivative works (such as those resulting from translations, adaptations or other changes we make so that your content works better with our Services), communicate, publish, publicly perform, publicly display and distribute such content. The rights you grant in this license are for the limited purpose of operating, promoting, and improving our Services, and to develop new ones."
About Google Drive:
[link]The Full Terms Of Service:
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Without this in their Terms of Service, Google Drive couldn't use your work in any way. They couldn't create thumbnails of your work that might interest people in buying your work. They couldn't translate any written work you have into other languages so it reaches more people. They couldn't update your webpage to HTML5, keeping your service up to date. They couldn't do... anything, really.
This allows them to use your work (and create thumbnails for web search, translate web pages so more people can find you, provide payment solutions for both buyers and sellers, store your work on their servers, and allow the people you authorize with the ability to retrieve that work.
In other words, this contract allows them to use your work in ways that you already want them to use it in.