When they are functioning correctly, water drains down the roof flows through the gutter screens/guard, into the gutter, then down the downspout, and onto the ground. However, debris in gutters can clog that flow and cause gutters to stop working and since the inside of your gutters are not easily seen, those obstructions can often go unnoticed.
Placing screens on gutters is one of the most common type of gutter protection, but unfortunately, it is not uncommon for smaller bits of debris (and particularly seeds and pine needles) to eventually plug those screens as well.
So, it may still be necessary to clean gutters, even those with protection. One of the better reviews of different types of gutter "guard" devices can be found at this website: http://www.dryhome.com/gutters-and-roof-damage-do-gutter-guards-really-work/
The cost of the materials and labor to install gutter guards can easily run into the thousands of dollars. And since you still need them inspected occasionally, well you still need to climb a ladder. That is why we came up with the Gutter Viper. To avoid ladder climbing and risk of falls, but still keep your gutter clean quite easily.
The Gutter Viper also works to clean debris the can sit or in the gutter screens or gutter guards. Pin needles, in particular, can clog screens and catch other debris Some of our customers us the Gutter Viper to clean the top of gutter guards. They don't have to clean as often, but they still need to clean them off.