The purpose of that question is to establish the basis in your Roth IRA in the event that you take a distribution from the account that may be taxable. If you have not reached the age of 59 1/2 and the Roth IRA is not at least 5 years old, then the basis is needed to determine whether the distribution is a return of your contributions (basis) or not.
To learn more about qualified Roth IRA distributions, see the following information from the IRS:
Qualified Roth IRA Distributions