The guarantee that Facebook won't sue you if they have patents that apply to React.js.

To put it simple, without the patent grant you'd be worse off. But with the Apache 2.0 version you'd be better off.

Not quite - there's an implied patent license that comes with licenses that mention nothing about patents, because otherwise nobody could use the code. Once the license explicitly gives a patent grant, it replaces that implied patent license. Think of it like C++ default constructors.

https://news.ycombinator.com/item?id=9113515