A planted space (aka "a garden") isn't a "space" without the sense of, a suggestion of, or the reality of, enclosure - regardless of scale; whether the scale is a 20X20' herb or rose garden or a 50-100 acre meadow bounded by woods or windbreaks. Just like a picture wants a frame.
I think that comfortable feeling is deeply embedded in the human soul, and it is the reason garden designers speak of outdoor "rooms." I kinda prefer designing or thinking about outdoor "hallways" - the paths which lead from space to space. Hallways, though, must lead to rooms or they have no meaning and no purpose.