My type could convert itself to an encodable value and use gob to encode that, but that requires writing extra code and creating both a bytes.Buffer and a gob.Encoder every time an instance of that type would be encoded, when we could just use the existing encoder.

One situation it would be useful is when you have a struct with unexported fields and you want to be able to serialize it but you don't want to make the fields public. I have a few of those, and just converting it into a new unexported struct at serialization time would be nice, instead I've had to manually encode / decode the fields and I've gotten it wrong a time or two. Another time this feature would be useful is when the struct cannot be serialized properly as it is and you need to change some fields or just run some code in general at serialization time. You can still encode the value by using GobEncode, creating a buffer, attaching a Gob.Encoder to it and pushing the changed value through, but that seems inefficient. One example is my Timer struct. It lets me check if a certain amount of time has elapsed and to do something if it has, for example printing "hello" every 2.5 seconds. And I want to be able to serialize and deserialize it, but I can't because it uses a time.Time to remember the last time it triggered and if I serialize and later deserialize it, for example while saving and loading a game, that time value will be relative to when it was saved, not when it was loaded. I fixed this by implementing the GobEncode and GobDecode interfaces, but it would be nice to have a better way.