The traditional reason given for using a joint groove is to allow the covering material to bend backwards in a more gentle manor, so that the board swings opens more widely without bunching up on itself. This was part of the reason the joint groove made a comeback in the early years of the twentieth century on leather bound books, along with the split-board structure, as a way to use thicker leather in order to make books more durable.
This, however, is only part of the story. Adhered-boards structures are very common in early nineteenth century American leather, paper and cloth bindings. Instead of laced boards, the slips are pasted to the outermost endleaf. Then the boards are adhered to this for about an inch or so. After this is dry, the unpasted area of the endleaf is torn off, and the boards can be trimmed to size.
As you can see in these images, the joint groove significantly affects the opening, even without any covering material. Note that it is the same book in both images, with the front board placed tight, and the back board with a joint groove.The difference surprised me. There is always something new to learn: this is what keeps me interested in the history and practice of bookbinding.