Add Leverage (Mortgage) and you greatly increase the ROI especially from the perspective of using Rents (other peoples money) to pay down the mortgage and increase your equity in the property over time. At this point then yes price appreciation is secondary bonus and we have an arguement of how and why Real Estate can be better than Growth Stocks in some scenarios and for some investors.
The first form of residual income, the leftover cash after bills and expenses are paid, is important when a person is growing their savings account or is seeking a loan or financing. When getting a loan, there must be an adequate amount of residual income available to ensure a person has the money to make the loan payments each month. If there is not adequate residual income, financing or loans will not likely be approved.
Residual income is the amount of net income generated in excess of the minimum rate of return. Residual income concepts have been used in a number of contexts, including as a measurement of internal corporate performance whereby a company's management team evaluates the return generated relative to the company's minimum required return. Alternatively, in personal finance, residual income is the level of income that an individual has after the deduction of all personal debts and expenses have been paid.