Initially function was introduced in r53541 (KAME initial commit) to "provide hints from upper layer protocols that indicate a connection is making "forward progress"" (quote from RFC 2461 7.3.1 Reachability Confirmation).
However, it was converted to do nothing (e.g. just return) in r122922 (tcp_hostcache implementation) back in 2003. Some defines were moved to tcp_var.h in r169541. Then, it was broken (for non-corner cases) by r186119 (L2<>L3 split) in 2008 (NULL ifp in nd6_lookup). So, right now this code is broken and has no "real" base users.
Even if ability to provide reachability hint from upper-layer protocol like TCP might sound as a good idea, it is not trivial to do efficient implementation. Personally I don't think it is needed ATM - if one really needs to reduce number of ND confirmations, "basereachable" variable can be increased to sufficient value. Given that, I'd like to remove the part of code in question.