100% meat would be a burger patty... in German this would be
Kartoffel-Hackfleisch Frikadellen or in short
Kartoffridellen, while the potato starch cause them to be more crispy on the outside and more juicy on the inside, since the water is being bound by the starch. Generally it's a cheap filler which most people won't notice, unless reading the fine print. They might not use breadcrumbs as binding agent (as the traditional recipe has it), because of the Gluten intolerance some people have developed - so that they won't need an extra Gluten-free version of them, which makes selling them a whole lot easier; that's "one size fits all", but people with potato allergy still cannot eat them. The recipe might be adjusted so that the biggest percentage of people can eat them.
Convenience & fast food often contain ingredients more concerning than potato, eg. MSG (which tricks the brain into believing it would be more Protein) and other agents. For comparison, try self-made meatballs or from the local butcher's.