Didn't really see an answer to this while skimming back through the thread.
I think it's two-fold: 1) to see the actual income the major candidates have and 2) to see the tax burden and charitable contributions the major candidates have. In the specific case of Trump the reason it is such a big issue is because he has consistently claimed to be a great philanthropist and businessman, so people want to see how much money he's actually bringing in (and has brought in) and how much money he's actually donated personally to charity. Now, we have a pretty good idea that the latter is very minimal due to reporting efforts of Fahrenthold of the Washington Post (has called over 200+ charities who have claimed Trump personally gave money or Trump/someone for his campaign claimed he personally gave money to and revealed exactly one personal donation over the past six years) and I expect that he isn't making that much personal annual income (far less than he wants you to believe) nor is he paying very much in taxes.
All of this is a pretty good indication of why Trump will not release his tax returns and it's comical that he and Pence continue to peddle this fiction of "Trump will release the returns when the IRS audit ends." Of course nobody knows the extent of the audit, when it began, when it ends, and I suspect it will never "end" at all.