Engineered floors are required in regions where humidity is an issue. For example, Utah is particularly dry whereas Florida is overly humid. Engineered floors are highly recommended in both of these areas. The physical conditions of the install area must also be taken into consideration. By and large, Solid hardwoods cannot be installed below grade (moisture issues) and cannot be installed over concrete (often found below grade). In the past, home owners were resigned to installing laminate or tile in their basements due to concerns over moisture and the inability to install over concrete. The look and feel of real hardwood was not an option.
Engineered hardwood overcomes these obstacle and can be installed below grade and over concrete subfloors using a glue down or floating installation. That said, it is never a bad choice to choose an engineered floor over solid regardless of the area; generally less expensive than traditional solids, it is a cost effective way of erring on the side of caution.