Me and my friend were pretty serious about creating a new data structure called "drum". A drum is a one way store. You write to it but can't read from it. We put it off till we figured a practical use.
An append only log? The value of writing data is that you can read it, otherwise your structure is semantically equivalent to not writing at all.
Or reading some information computed from it, I suppose. Something needs to read it, ever, for that of course, but it doesn't necessarily need to be exposed in the interface.
Backups? People tend to treat them that way anyways.