Gutter wedges are used when the fascia board is angled or canted and is not perpendicular to the ground. This happens when the roof framing members (the trusses) are not cut on the ends to match the angle of the pitched roof. Gutter wedges will "build out" the fascia to push the bottom of the gutter outward which allows the gutter to be installed perpendicular to the ground. There are a few different hanger types that allow the gutter to be suspended from the roof which are also used for this application but the wedges make it possible to install the gutter without having to interfere with the roofing materials.
Gutter wedges are available for k style gutters and half round gutters and available in aluminum, copper and galvalume.