Sephardic Halakha - Rambam Hilkhot Tsitsit 1

Lift up your heads, Oh charitable ones!

...Hashem showed Moshe a coin made of fire, a coin that was all quality and not quantity. Hashem's message to Moshe was that the half-shekel was not about showing off how much money one could give to charity. Rather, it was about each person making his contribution and showing that he counted in a meaningful manner. Each person's contribution was special, and when a person does something special, a person can be proud and can life his head up high. Thus, Hashem told Moshe to "lift up the heads" of the Jewish people. He commanded Moshe to teach the Jews a mitzva that would allow each person to feel pride in his role as an equal member of the Jewish nation.

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

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