A write-only file does actually have a use. It could be an output device file, which could be mapped to a kernel device driver for a piece of hardware that was only an output and provided no feedback.
In practice, all output devices in common use have some kind of feedback, and so most device files are read/write, but that isn't necessarily the case.
Of course, such a file would normally be b-w--w--w- or c-w--w--w- , but since the advent of procfs, it would be perfectly possible to create such a file under a virtual filesystem like /proc or /sys. On my system, for example, I have a file named /proc/sysrq-trigger with permissions of --w-------- (but none writeable by all users).