Gutters must be cleaned out for water to properly flow into the downspouts, and ultimately your drains. Leaves, sticks, grasses and other debris can cause gutters to clog and overflow, thus leading to eve and foundation damage. In addition, your drains can backup if excess debris clogs them. Your insurance company may also refuse to honor your claim if they can prove that your gutters have been neglected.
Your gutters should be cleaned thoroughly at least 2 times per year. Additional cleanings may be necessary if heavy tree cover surrounds your house.