A linear regulator solution is very simple and low cost. Simple means few external components and more board space. They typically have the lowest noise of all the types of power supplies and can be used when the Vin to Vout ratio is small. In applications where this ratio is small, the efficiency will not usually be as critical. In summary, linear regulators are useful in low to medium current applications where output ripple, space, and cost are very important to the customer.