If you know you are heading down the path to divorce, and are looking for evidence so that you can terminate the marriage, with some sense of financial stability, maybe tax returns are not where you should be looking.
People are motivated to do all they can to minimize their income for reporting to the IRS. Don't mistake that for us saying that we suggest you lie to the taxman about your income, because that's not the truth! However, people are incentivized to show maximum income on credit applications. Having copies of credit applications can be evidence that your spouse has understated their income.
The relative income of one spouse to the other can impact everything from spousal support (alimony) to child support. Proof of income, especially if it is much different than stated on tax returns, can be beneficial. So, if you are making plans to exit your marriage, and happen to see credit applications that show more income for your future ex., it might pay to get copies.