There are several reasons to make sure your gutters are cleaned out. Gutters are installed in order to catch the water falling off of your roof and redirect it away from the foundation of the house which could cause serious damage.
Gutters need to be kept free of debris to be able to drain properly. Too much added weight can pull the brackets loose and damage the gutter system.
If your gutter is not draining correctly, that water will freeze in the winter months and could potentially cause the gutter seams to open up, creating a leak.