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Starch indicator added at the END (iodine titration)

Hello, I stumbled upon a question involving titration of iodine with sodium thiosulphate and one of the question asked why the iodine was added towards the end when the reaction was nearly complete. I know that there is a side reaction, but I just can't remember it.

Here's the full question

''To determine the composition of a sample of brass, 1.5 g of the alloy was treated to give 250 cm^3 of a neutral solution of copper (II) and zinc nitrate.

Excess potassium iodide solution was added to 25cm^3 portions of this solution and the liberated iodine titrated with 0.1moldm^-3 sodium thiosulphate reaction.

2Cu (2+) + 4I (-) > 2CuI + I2
2S2O3 (2-) + I2 > 2I (-) + S4O6 (2-)

Explain why the indicator is not added until the reaction is nearly complete (1 mark)''