A bit of background here: All of the religious festivals were centered around the Temple in Jerusalem, three of which were mandated by Scripture for every adult male. The Jewish (v. Roman) ruling body convened on the Temple grounds, the tangible glory of all Israel was centered in the gleaming gold and limestone of its walls; indeed, even the top of the Temple had gold posts spaced around the top to prevent bird doo from sullying its holiness.
There were several parts to the Temple and its grounds: The Court of the Gentiles, where anyone was permitted, regardless of religion; the Court of the Women, where every Jewish person was permitted; the Court of the Israelites, where only Jewish men were permitted, primarily to watch their sacrifices slaughtered and burnt; the Court of the Priests, which was restricted to priests; the Holy Place which was reserved for select priests; and the Most Holy Place, which was the dwelling place of God’s Presence and permitted only to the high priest, and once a year at that.
The old Temple probably stood where the Dome of the Rock, a Moslem shrine, stands today. The grounds were probably flat and empty then, except for the Temple building. Pilgrims would have thronged these courts back then, bringing their sacrifices to the Temple and milling about for the festivities.
Maintaining such a large space cost money, and the annual half-shekel tax was a major contribution to maintaining the many courts, purchasing the the wood, incense, salt and other items affecting the sacrifices, paying Temple workers, etc. There were additional costs associated with the festivals; for example, just before Passover all of the graves were whitewashed so pilgrims didn't become 'unclean' by stepping on them.
This was the tax being collected when Jesus ordered Peter to throw out a fishing line; the first fish caught would have a coin in its mouth.