Route Adver­tise­ment allows zero-configuration, i.e. auto­mat­ic address set­up with the cor­rect pre­fix and a suf­fix tak­en from the MAC.
OTOH rtad­vd is the serv­er part and you cor­rect­ly stip­u­lat­ed that you do not need it; not unless the that FreeB­SD host is also an IPv6 router, e.g. one of his inter­faces act as gate­way to anoth­er local net­work in a dif­fer­ent (or longer) pre­fix.