Gutters is a good place for vegetation to grow.
Plants can result for your gutters to become so clogged and impossible for rainwater to flow to the downspouts. When that happens, the gutters can overflow, which can cause damage and alot of money.
Seeds can fall or get blown into gutters or can be carried by birds and other animals. If gutters are also filled with decaying leaves and other debris which can serve as a good compost for the seeds to germinate and grow.