They certainly served a purpose 10-15 years ago. With modern equipment, however, there's really no need, unless your draw length is exceedingly long (i.e., >34").
I shot an overdraw for years in an effort to decrease my arrow length and thus to increase my arrow speed. Unfortunately, this type of setup was (and still is) very sensitive to imperfect tuning and or form (especially with disturbances in your grip).