Padding the running time.
There isn't an answer that works for me. Ripley wasn't a fighter till after the confrontation on the life boat (and she is mostly a fighter in Aliens because the xenomorphXenomorph has taken her life and her daughter from her). I think it was a left over plot thread probably from a earlier version of the script and they left it in because it fuels tension in the movie and adds some suspense and drama near the end.