Repeating elements or even certain properties of elements can produce many different effects, but repetition fundementally creates unity in a very direct way. For example, repeating certain hues creates color unity between elements; repeating elements of a typeface design creates textual unity; repeating proportional relationships between forms creates formal unity; etc. In other words, using elements and properties consistently throughout a design is, in and of itself, a form of repetition. The unity one achieves with this kind of consistent repetition creates thematic structures, and these themes can be used to unify multiple compositions.
Speaking of themes, notice what repeats and what varies in the below example:

Repetition and Rhythm
Repetition also provides an excellent way to create rhythm within a single composition. Below is a photograph of Jackson Pollock's 1952 piece Blue poles which, despite its overtly erratic presentation of elements (in this case paint drip lines), has a repetitive structure of thick, near-vertical, blue lines that sits atop its fragile, visual hierarchy. Thick, near-vertical, and blue: these are all design attributes that, when repeated, create rhythm and a recognizable pattern. Take those consistencies away and this piece might just look like an accidental mess. Add them, and this piece suddenly has intention.
