Roth IRA withdrawls and basis calculations

I have a Roth IRA that I last contributed to in the early 2000s. I also converted a balance from a traditional IRA in that same time period. I am now over 59 1/2 and took my first withdrawal from my Roth last year. I received a 1099-R, with a distribution type of 'T'.

 

Going through Turbotax to report this withdrawal, I get a lot of questions about the basis. Since my contributions and earnings are all tax-free at this point, I don't understand the necessity of going back through all my records to compute the basis. Can someone explain to me the benefit of doing these calculations for someone in my situation?