How can we appreciate what we cannot hear? There was an interesting study done by a music psychologist indicating that people find repetition more pleasing than lack of repetition.
In her study, the psychologist used the music of Luciano Berio, an instrumental composer famed for creating dramatic compositions by avoiding repetition, to test her theory. Berio's compositions were akin to a continuous stream of melody, each segment different from the last. So she copied segments and repeatedly pasted them over the length of a song and found her version was much more popular than the original piece.
I believe this is simply because the human brain finds non-repeating melody too chaotic. Even though you may be listening to the music, you cannot hear it. Those combinations of notes, chords, and rhythms in multiple contrasting, harmonious, or jarring juxtapositions can only be processed when repeated multiple times, allowing our brains to examine each detail.
How can we appreciate what we cannot see? Like symphonies, gardens can become very complex and intricate compositions. Juxtapositions of colors, textures, shapes, and habits can combine together in a dizzying variety of effects.
Repetitive patterns help break down seemingly chaotic scenes into tiny, digestible chunks for our brains to interpret, allowing us to not just look, but truly see a garden.
Not only is repetition helpful, it can very well be necessary for certain designs to be widely accepted. A chaotic garden design can generate a feeling of uncomfortable uneasiness. I suspect wildflower mixes and randomized plantings have suffered from such rejection many times in the past due to this simple fact. "I don't see it, ergo, I don't like it".
A little repetition goes a long way. Perhaps its not exactly the same texture or color, but subtle similarities can pull a design together and make it magic. So give me rhythm, give me rhyme, and make my garden sing!