Half Shekel rabbinic law applicable even for paupers, why is it so important?

... A third question is based on Rambam (Hilchot Shekalim 1). He says that it is a positive commandment for one to give the half-shekel, and it is so important that a pauper must sell the clothes off of his back in order to fulfill this law. This notion of selling the shirt of off one's back appears in two other places - in order to buy four cups of wine for the Pesach seder and in order to buy Chanukah candles. It is understandable that both of those commandments are reflective of great national miracles, and thus it is not surprising that we would require one to go to great lengths to be able to participate in them. However, how does the giving off the half-shekel fit in? Why is it so important that one has to sell his shirt in order to participate in it?

http://www.chaburas.org/shekel.html