In many standard homes, the exterior corners of rooms are cooler in the winter (or warmer in the summer) than the middle of the wall assembly. Why is this? The concept is known as "thermal bridging." Wood is more conductive than insulation, which means it is better at transferring heat or cold air. When a home has wood that extends from the interior wall all the way through to the exterior of the building shell, it acts as a "bridge" for cold (or warm air in the summer) to transfer from the outside to the inside. Thus, in our homes, we want to minimize the thermal conduction of the walls.
Most contractors just insulate between framing studs, which leads to many thermal bridges in a wall, particularly in the corners where there is the most framing. We can minimize thermal bridging by using continuous layers of insulation that encapsulate the framing of a home. This practice is more expensive, but necessary to minimize thermal bridges.
In addition to the conductive nature of wood, there are often gaps in the framing that allows for cold air to enter a home, rather than through conduction. Most contractors still don't air seal cracks in framing as it is a tedious and detail-oriented process. However, all those cracks really add up and allow for your conditioned air to escape while allowing unconditioned air to enter your home. Thus, it's critical to spray foam all the small cracks in the framing to prevent air leakage in addition to mimimizing thermal bridging.