Over the course of time, an organic muck builds up creating a medium for growing all kinds of stuff, resulting in the addition of weight to the gutters making them vulnerable to getting separated from the fascia boards and collapsing
Dirty gutters with clogged down-spout can create a host of ills. Water will overflow in all directions, causing fascia boards to rot and erosion problems on the grounds. Sometimes water makes away in between sidings inside the house and basements.
It is recommended to clean the gutters at least twice a year, making sure all downspouts are unclogged.