The purpose of a gutter is to carry water away from a building to a good drainage area. If gutters are not cleaned, material will build up in the gutter channel and water will overflow onto the building and foundation. Clean gutters by scooping out debris and then rinsing the gutter with a hose.
If gutters are not cleaned and water overflows from the gutters, it will fall along the foundation of the building. This can soften the soil under and around it which will lead to foundation cracks. Also, if the foundation is wet and then freezes, this will cause heaving and cracks. In extreme cases, water damage can cause structural failure in a building.
If gutters are clogged, water will overflow onto the building and also will be trapped in the gutter after rain. This moisture will cause rot in wood fascia and soffit boards (the wood near where gutters are attached). This wood rot can spread to adjacent areas, such as the attic, and also provides an easy point of entry for rodents.
If gutters do not drain properly, this will contribute to ice building up in them during the winter. This will keep melted snow from draining off the roof properly and will keep that water and wet snow against the house. Often, that water will migrate into the house and result in ceiling leaks, wall leaks and moisture in the attic.
The debris in gutters, especially when moist, is a very attractive habitat to insects. These insects will be attracted to the inside of the house when the weather changes, and so can lead to insect infestation within the home. Also, wood-eating insects such as termites and carpenter ants are attracted to rotting leaves and branches that collect in gutters and so not cleaning gutters can lead to an infestation of those damaging pests.
Just as clogged gutters can damage a building's foundation by allowing water to collect close to the base of the structure, this water can also infiltrate basement walls. When water collects in large quantities against a basement wall, it is almost inevitable that it will find its way into the basement. If water does leak into the basement, this can cause black mold, smells, water damage and flooding.
The water that falls from obstructed gutters can kill plants and grass living below. When those plants die, their roots no longer contribute to holding the soil in place. The water will then wash away exposed topsoil and sand. If left alone long enough, this erosion can etch away the landscape under the clogged gutter and leave a ditch.