I'm rather sure he was able to strum along on it while singing religious songs.
During the Hippie era in the 1970s, a large number of protestant revival movements sprung up all across Europe. While these movements didn't agree with many of the values and ideas of the Hippies (that is, most of them opposed smoking weed and the more conservative ones were adamantly against free love), they still tried to copy the look and feel of the Hippie movement because it was the cool and modern thing to do. As Hippies are known for playing guitars or ukuleles to accompany their singing while traveling the world, the protestant revivalists adopted this type of instrument too. Günter probably started out in this social environment.
Even today, in Germany there exists the well-known stereotype of the young and hip protestant preacher who wears sandals and other Hippie-inspired articles of clothing, likes to sit around campfires with teenagers while playing happy Jesus songs on his guitar and spouts sentences like: "Hey y'all, know who's cool? Jesus is cool, cuz he's my coolest homie!! Jesus is one hyper-awesome kewl space cowboy!!!"