Structure in music is often a difficult topic due to the varying amount of forms and different applications of them. Structure in music does not only include form, but all of the elements that make up music, such as texture, instrumentation, dynamics and the such. The solution below briefly describes the application of this and lists different widely used compositional forms, as well as a few references that will aid in understanding these specific forms better.
To answer the question directly and succinctly, repetition and form in music create predictability in two different ways. Repetition creates predictability in the sense of anticipation and familiarization. When someone hears something repeated, it gives them a level of comfort that is simply human nature. This comfort causes anticipation, which allows the listener to identify with the music.
Contrast also creates predictability by giving the listener variety and causing the listener to recall the thematic material that is being contrasted, which causes anticipation of the thematic material.
Repetition and contrast are not limited to thematic material. They can also manifest in dynamics, texture and timbre and be used in the same way as described above. All of these elements combined cause structure in music: form, dynamics, texture and timbre.
In the analogy of a building: formal structure (form) is the foundation; texture could be considered the walls and other internal structures and timbre, dynamics and all of the other details involved in music could be considered the decorations and the details that make the music interesting.
In terms of form, there are specific forms that I will attempt to define, and I'll include a few ...
Structure in music is exemplified. References are also provided to further validate the findings.