This sysctl interface to set a socket option will be used by an upcoming tool `tcpsso`, which will allow to set a socket option via the command line.
This depends on some upcoming changes to `tcp_ctloutput_set()`, which allows it to be sent from a context different from `setsockopt()`. It is under review in [[ https://reviews.freebsd.org/D34164 | D34164 ]].