I found out https://discussions.apple.com/thread/5222408?start=0&tstart=0You can rotate inserts and annotations, like text boxes, rectangles or signatures, by arbitrary angles:
ROtateHold the Command ⌘ key and rotate the selected insert or rectangle or whateverannotation with your 2two fingers on the touch pad while pressing cmda trackpad.
Now whyAs far as I know this cannot be found in the Preview Help sectiononly works with trackpads and is another Microsoftismundocumented. It works on macOS High Sierra.
This is from an apple communities question: How can I rotate the text box in Preview