Protocol preferred feature:

It enables okhttp connect to a http(without TLS NPN/ALPN) based spdy host.

This feature is meaningful and convenient for some web service refinings with spdy protocol,
actually we already used this feature in the nginx proxy layer of our mobile web service api,
it works and thanks for your guys' awsome works in okhttp library.
Another aspect, it can unify the web service request facade such as Andriod and iOS, since NPN
"this extension isn't supported by Secure Transport (Apple's TLS implementation)".
One example for iOS request: https://github.com/twitter/CocoaSPDY

You can check the usage of sample in this path and sorry for my poor English:
samples/guide/src/main/java/com/squareup/okhttp/recipes/ProtocolPreferredRequest.java