There are many prospects for home buying that have an income to afford a mortgage, but can't meet the standards of full documentation.
Full documentation generally requires that applicants show that the income they claim was actually earned in each of the two prior years. This is usually done by presenting W-2s or tax returns for two years. Self-employed borrowers usually have the most trouble meeting this requirement, and stated-income loans were originally designed to deal with them, but other legitimate cases quickly emerged.
Many applicants with incomes from salaries can't meet full-doc requirements. They may not have held their position long enough, or their latest increase in salary may not be reflected in documents covering past income.
If a married couple pools their incomes and one has a much lower credit score than the other, the full-doc rule is that the lower score is the one used. Stated income allows the partner with the higher score to claim all the income, which appears reasonable in most situations, especially in community property states where husband and wife share legal right to each other's incomes.