It's not about knowing the income sources. It's about knowing how those income sources fit into the tax rules. Put differently, for the purposes of taxes, your money has a kaleidoscope of colors, with different colors incurring different tax rules, and most people are to a degree colorblind when it comes to their money's colors. Not completely colorblind, but trying to work out if your color is mauve or magenta, for example.
The solution to this of course is to simplify the tax code so that you don't have so many colors to your money. I very much would love to see this happen, but it will produce howls of protests from every special interest group who benefits from special tax advantage to their interests.
Whether or not you do your taxes by hand, you should probably have a good idea of your income sources.