A soffit protects the exposed overhang area on a home's eaves to keep weather elements and pests out of the area. It prevents infestations in the attic and damage to the rafters. The soffit also serves an aesthetic purpose by giving the eaves a finished look.
Without soffits, the rafters of the roof would be fully exposed to the outdoors in the area that overhangs the side of the house. By covering the open area with soffits, bats, squirrels and other animals no longer have easy access to the inside of the home. Water and moisture from weather elements also stay out of the attic and away from the rafters to reduce mold and rotting that could otherwise occur. Ventilation built into many soffits helps promote circulation in the attic to further reduce moisture buildup and keep the house cool in the summer.