Because it rains and rain is consistent and persistent in trying to infiltrate your home and if not managed effectively potentially undermining the foundation of your home.
Rain Gutters are designed to catch the rain from the roof and channel it safely away from your home before it falls to the ground, often splashing organic matter back onto the wall of the structure within 1-2 ft of ground level. If shaded, or left wet or generally untreated this can lead to array of of problems no homeowner wants to encounter for example mold, mildew and rot.
The nature of rain is such that once these problems emerge, they will continue and worsen over time. Water damage in these situations is not self correcting.