It’s an accepted custom that Japanese women office workers looking for husbands willingly accept menial jobs in corporations. Theirs position is seen as temporary. Koshikake means a chair, a bench, or a stool; an object upon which one rests briefly. It’s easy to see how the phrase would come to mean a stepping stone to the next stage of life (most frequently wife and homemaker).
A: Jinjika no Kanai-san, iyoiyo kekkon da tte.
B: Hee, yokatta wa ne. Doose koshikake de tsutometeta n da kara, hayaku aite ga mitsukatte.
A: Did you hear Ms. Kanai is getting married?
B: Really? That’s good. Her job here was only a stepping stone anyway. Good for her to have found someone to marry so quickly.