A strange wee story which ties in to the Gospel reading for Sunday past.
The tax discussed was the Temple Tax required of every male Jew over the age of 20 for the upkeep of the Temple in Jerusalem – a very costly business.
The writer has included this passage because it is a reminder that to be enabled to not only survive, but thrive, the early Christians had been seen to be good citizens and pay their taxes. When this gospel was written the Temple had indeed been destroyed (AD 70) by Vespasian. He had instructed the tax should be paid to the treasury in Rome. To refuse to pay would have invited retribution so the gospel writer is telling them they must pay taxes due of them.
The story essentially says, Peter just get on with your job (as a fisherman) that you can earn enough to enable you to pay the tax.