Roof gutters are on almost every building, but many people don't think about those gutters at all, often neglecting them or not even installing new ones when the old ones fail. However, the gutters are essential for protecting both your house and the land around it. Without gutters that are installed properly and in good shape, you can face several problems down the road.
The basic purpose of gutters is to catch rain before it runs off the roof in a waterfall and to direct that rain to a downspout. The downspout directs the water to a drainage point away from the house's foundation. Without a gutter, the rain can run off in sheets anywhere along the edge of the roof.
Having constant streams of water falling off the roof is not good. Landscaping that is under the edge of the roof will drown, and the land will turn into a muddy mess. If the water lands too close to the walls of the house, the soaked soil can eventually damage the foundation, either due to erosion and shifting soil letting the foundation move, or because there's a crack in the foundation and the water is leaking in, causing mold growth.
The land around your house should be graded so that water runs away from the house, but even with perfect grading, water-saturated soil that is right next to the house can still cause massive problems. This is why the lower ends of downspouts are curved away from the house so that all the water rocketing down the spout will flow to a point farther away.
In addition to having just the gutter, there are a couple of accessories that may be necessary. One is a leaf screen or grill. This is a mesh or metal screen that sits on top of the gutter. The screen is supposed to exclude leaves and twigs from the gutter -- if those clog the gutter, especially when the leaves dry, water will flow over the sides of the gutter and cause the aforementioned saturation problems. Note that leaves can dry on top of the gutter screen and prevent water from getting in, so the addition of a screen does not eliminate the need to clean the gutters.
Another item is a corner baffle. If your home has roof valleys, where two sections of roof meet to form a V, rain can run off both sections of the roof and into the valley. The combined streams create a river of water shooting off the roof and over the gutter. A baffle is a small piece of material that extends the height of the gutter, blocks the flow of water over the gutter, and diverts the runoff into the gutter on either side of the corner.
If you live in an area with low rainfall and where storms are typically weak, a barrier that runs along the edge of the roof may work. These are L-shaped items that sit at the edge of the roof; rain runs down the roof, hits the barrier, and is directed to a smaller gutter and downspout at the end of the barrier. However, this is not suitable for areas where there is a lot of heavy rainfall or for buildings near trees that shed leaves.
If your home does not currently have gutters, or the ones you have are old, get them replaced now. The sooner you have strong, well-made gutters on your house, the sooner you'll see improvement in landscaping conditions and house foundation health.