why write now, when you can write later?
Yadon is a struct which implements Write + Seek but doesn't actually write anything until later, when you call .apply().
~~yes~~ I was trying to push a generic write operation using binrw through a channel to be actually performed on another thread, and being able to store the result of the write operation meant I sidestepped some particularly hairy issues where I would have had to store trait objects which had an associated generic function - impossible since having that associated generic function made the entire trait not 'object safe'.