Over the course of time, an organic muck builds-up creating a good medium for growing all kind of stuff adding weight making gutters vulnerable to get separated from it fascia boards and collapse
Dirty gutters with clogged down-spout will create a host of ills. Water will overflow in all directions causing erosion problems and rotted fascia boards. Sometimes water makes its way in between sidings inside the house and basements.
It is recommended to clean the gutters at least twice a year, making sure all downspout are unclogged.