Not all homes need a gutter system. Gutter systems may not be needed in an arid climates and areas not subject to periods of heavy rains.
The rest of us need a good system to collect the storm water that lands on our roofs and directs the collected water away from the house. We need such a system in order to help reduce water and moisture damage to our homes. The wetter the climate the more we need a properly functioning gutter system. Water that collects next to the foundation of a house will tend to find its way into some part of the structure and that can cause water and moisture damage.
Modern homes are designed to conserve or "trap" energy. That's good. But in the process, such homes can also trap moisture and that can cause big problems and costly damage. For example, if the soil around the foundation is soaked from roof drainage water, we can assume that at least some of that moisture will find its way into the basement, crawl space or the concrete floor. Some of this moisture will then be absorbed by the air in the home. If this occurs in the winter time, we can assume that the warmed air in the home will absorb a great deal of moisture (warm air can absorb more moisture than cold air). If this moisture saturated air finds itself into some spaces with cold surfaces - and it will, then some of this moisture will condense against these cold surfaces. This can happen at windows, attic plywood and inside exterior walls, etc.
Condensation at windows may just be bothersome, but in attics and walls it can cause fungal wood-rot and that can be very expensive to repair. A good gutter system helps to control the amount of water and moisture that gets into a structure. And that, in combination with various types of venting system, can prevent the damage from trapped moisture.
The Roof - It takes a well designed roof to drain all of the storm water into the gutters or roof drains.
The Gutter - The gutter (or roof drain in some flat roof) needs to be designed to capture all the roof drainage and direct the water into the downspouts.
The Downspouts - These are the pipes that carry the water from the gutters to the drain system.
The Drain System - This is the in-ground set of pipes the leads the water away from the structure and into a storm drain or on site system.
All roofs and gutter system will need some maintenance, but with proper planing it is possible to reduce that chore to a minimum. Here is how: