Copper 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.
The copper wedges also can be used, at times, when there is no fascia and instead there are only exposed rafter tails. In this instance the rafter tails would need to align with the roof deck so that the gutter would mount properly so that it would catch all of the water running off the 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.
Copper gutter wedges are available for k style gutters and half round gutters. The wedges are recommended to be installed on the same spacing as the hidden hangers, which are recommended every 24" to 32".