Michigan Ms. 136, vellum, 4th-6th century, Egypt
The seven Greek vowels played an important role in Graeco-Egyptian ritual magic. They were understood as higher powers and as such often invoked for protection and healing. The increasing and decreasing triangles seem to have been a magical means, I currently think that they were used to call the vowel-powers so that they could take the illnesses away with them when they leave. Illnesses were believed to be caused by evil demons. The decreasing triangles would represent the leaving of the vowel-powers while increasing triangles, which are attested as well, would represent the incoming power of the vowels growing stronger.
Photo of the manual: Michigan Ms. 136, University of Michigan Library, Ann Arbor