The secure-random is designed to find window.crypto and use it (you guessed it, Internet Explorer does not use the standard location) .. This is an HTML spec for providing:

cryptographically strong pseudo-random number generator seeded with truly random values

If the browser does not provide window.crypto, a key will not be generated. Because there have been poor implementations of window.crypto in the past, (mostly as Bitcoin was coming of age), we do what most people do and combine the data with the other sources described above and hash it all together to create the key.