$100 spent pre-tax simply means $100 from your paycheck is not taxed. Most employees in the US see their paychecks taxed at about 35%. So, by spending $100 pre-tax on eligible commuting costs, you will save the $35 you would otherwise have paid in taxes like FICA and Social Security.
Pre-tax spending allows you to pay for certain expenses before taxes are applied. In effect, the amount of your paycheck that taxes are applied to becomes lower, and your take-home pay increases. It's a win-win!