[jhumphries131]

The key "killer" use case is for describing RPC schemas. By describing the schema in an IDL, you can then generate client stubs and server interfaces in a variety of implementation languages, allowing interop between heterogenous clients and servers.

They are also useful for describing domain models. This isn't surprising since domain models usually find their way into RPC schemas (since RPCs will often query or define model data). But they can also be used in other cases, such as for persistence and structured logging.

Some misuse I have seen involves trying to make a protobuf model the _only_ representation of a domain model: it is almost inevitable that a physical model (a representation of your data in a SQL database, for example) will need to vary from a logical model, and trying to make a single protobuf representation serve double duty can be a source of problems. Making protobuf schemas conform to physical storage constraints often makes for worse abstraction. The model becomes de-normalized (which can make constraints and relationships harder to model/enforce), and can even leak details that consumers shouldn't know about or care about.

Another misuse is using it for data that never leaves a process -- a program's private, internal state. If a data structure never needs to be serialized (to persist or send to another process), then you're better off using native data structures in the implementation language (which generally have far greater flexibility/expressibility as well as better performance).