The interest rate on a loan tells you how much interest accrues on the outstanding balance. APR goes further: it folds in origination fees and other upfront charges, spreading them across the loan term to produce a single annualized percentage that represents the loan's true cost.
Two loans with the identical interest rate can have very different APRs if one charges a 3% origination fee and the other charges none — APR is what makes that difference visible and comparable, rather than requiring you to do the fee math yourself for every offer.
Short-term lenders sometimes quote a "factor rate" instead of an interest rate (e.g., repay 1.2× what you borrowed). Factor rates look deceptively small because they aren't annualized — converted to APR, many factor-rate loans turn out to carry effective annual rates well over 40-50%.