[bchallenor]

To support schema evolution. Of course it's up to you to write code that can actually handle missing fields, but once you have a required field in a message, you can never remove it.

From the docs:

"You should be very careful about marking fields as required. If at some point you wish to stop writing or sending a required field, it will be problematic to change the field to an optional field – old readers will consider messages without this field to be incomplete and may reject or drop them unintentionally. You should consider writing application-specific custom validation routines for your buffers instead. Some engineers at Google have come to the conclusion that using required does more harm than good; they prefer to use only optional and repeated. However, this view is not universal."

https://developers.google.com/protocol-buffers/docs/proto