Back in my day, and to a lesser extent still today, many people named their children after themselves. Kenneth Schroeder has a son named Kenneth Schroeder, or Susan Marciano has a daughter named Susan Marciano, and it can get pretty confusing. Middle names help them sort it out. My father and I had the same first and last names.
Today with the explosive spread of communication via the internet, we all encounter people with the same name. If you're in business or the arts or academia and you want people to be able to find you with a search engine, you'd better use your middle name. There are three other Fraggle Rockers out there, and each one of them has just enough in common with me that it would make a prospective customer, fan, or correspondent wonder whether they'd found me. Two of the three are significantly more prominent than I am so it takes people a while to track me down. It's too late to add my middle name now because no one knows it. Back in high school people knew me as Fractionator Pythagoras Rocker because my father had already coopted the nickname Fraggle, but I'm not about to try to reestablish myself with my full name.
Of course some communities have found more creative ways around this problem by breaking out of the paradigm of traditional names and giving their kids names like Condoleezza. But if you're not going to do that, I advise everyone who is going to have children to give them a middle name so they don't get lost on the internet!
Japanese traditionally only have a surname and a given name, in that order. But between them they typically have seven or eight syllables so there's not a lot of overlap among Japanese names. Japanese-Americans, at least back in my day, wanted very much for their children to grow up and assimilate, so they gave them English names. But they also wanted to be able to write about their kids in their letters back to the old country and there were two big problems. One is that English has about eleventy times as many phonemes as Japanese, and the other is that in Japanese every consonant must be followed by a vowel (except N). It's impossible to transcribe names like Martha and Stanley in kana, the phonetic syllable-alphabet. So they gave their children Western middle names carefully chosen for their conformance to Japanese phonetics, and many nisei had names like George Ben Matsuhara and Alice Naomi Hasegawa.
Chinese only has 1,600 distinct syllables (in Mandarin) so if people only had two one-syllable names it would be chaos. (Although that has become more common with the one-child rule.) Every family has a log book that was passed down from their great-grandparents, stipulating the "generation name" for each generation of offspring. Surname comes first, then generation name, then finally you get to choose the third name for your kids. My Chinese girlfriend was born just as Japan was overrunning China, and her generation name happened to be a word that means "to remember fondly." So she and each of her siblings got a third name that was the name of one of the provinces that had been captured by the Japanese. Her name meant "I miss Liao(ning) province."
In the American South, it's common for adults to form nicknames for children based on both their first and middle names, something that is absolutely not practiced elsewhere. That's why Southerners so often have names like Billy Bob and Suzie Jo, from William Robert McCoy and Susanne Josephine Thibodeaux.