Short Answer
Gutters collect roof runoff and direct it away from the building. They are part of the home's water-management system.
Detailed Explanation
Gutters serve a critical function in the home's water-management system: they collect the water that runs off the roof and channel it through downspouts to a location away from the foundation. Without gutters, roof runoff falls directly next to the foundation, where it can saturate the soil, cause foundation cracking, leak into basements or crawl spaces, erode landscaping, and damage siding and trim from water splash-back. Properly functioning gutters protect the foundation, walls, landscaping, and exterior components from water damage. They're an essential part of the building envelope, not just an accessory.
Why It Matters
Gutters protect the foundation and exterior walls from water damage. Their absence or failure can lead to foundation problems, basement water, and exterior deterioration.