A trace amount of acid is used to keep the amine mostly in its neutral, nucleophilic form while still allowing protonation of the sugar's oxygen.

The acid is only needed to catalyze the ring-opening of the sugar, so a small amount is sufficient.

A small amount of acid is used to prevent the formation of side products by minimizing sugar degradation.

A trace amount of acid is used because the reaction is pH-sensitive and requires strongly basic conditions to proceed.