#1 A middle name distinguishes between or among those with the same popular first name. A small church with four Virginias needs a couple of them to go with Virginia Lou and Virginia Ann. A junior high classroom with three Ashleys needs help. Of course, the middle name doesn’t always work. There was my high school typing class where I sat across the aisle from another Virginia Ann. Our resolution was to tell the typing teacher that we both went by “Virginia.” If she called on “Virginia” for the answer to a question, whichever one of us knew the answer gave it. It worked for us.
#2 One can honor an ancestor by pairing a strong common first name with a rather exotic antique name as in Jack Rhesa Butler. 
# 3 The mother’s family of origin may be honored in a middle name like Mitzi Morrow Butler or Benjamin Taylor Butler.
#4 One may dislike one of the father’s names or object to the possibility of the child being called “Junior” as is common in the south or when Anthony Dinozzo’s father shows up on NCIS. A new name for calling may be chosen to go first with the more liked namesake name in the middle, such as Murray Allen Butler.
#5 Probably the most useful reason is the enhanced hearing ability associated with a middle name. Having moved to Caledonia MS for the year Al was to be in Korea, our five-year-old son immediately invited his counterpart from across the street in to play. I briefly interrupted their Lego construction to ask the little boy his name. “My name’s Bobby Brown,” he said. “Sometimes my mama calls me Bobby, but I can’t hear her so good. Then she calls me Bobby Ray Brown, and I can hear her real good.”
So there you have it. If you anticipate a child with poor hearing, you may want to adopt our Southern custom and bestow a middle name.