There are a few different reasons to combine various oils during the cooking process. It can enhance the functionality, infuse extra flavors, or even save money.
Start with a neutral base like canola oil for most cooking, then add small amounts of an oil with specific, intense flavors to change the overall flavor of the base oil.
The most common blended oil involves combining a little olive oil with vegetable or canola oil to impart much of the flavor of a quality of EVOO at a fraction of the price.