It was done because Mohammed was a pervert for whom having a lot of other wives wasn't enough. I guess in his old age he needed a new "twist" to make up for the lack of Viagra.
Those facts in no way excuse what we would term heterosexual pedophilia, of course.
But, back to the point - this was the Dark Ages, and Mohammed was fortunate enough to live in one of the few places not depopulated by one of the world's greatest plagues (just then past), and all of these events took place in a small population that depended on trade with guys who chased sheep and goats.
As everyone in this particular thread knows, 100% of the information we have about Mohammad, or from Mohammad, comes from people who wrote his words, and the words of others, and their own thoughts on small pieces of wood or leather, or old crockery, with charcoal sticks, or pens dipped in inferior quality ink! Some of this material ended up in the Koran; some was relegated to the Hadiths, and yet other of it was just left for historians to find and sort out.
Those who presume Divine guidance behind this writing and sorting and categorizing would argue that all the material concerning the age of the last wife is absolutely true.
Those who question the legitimacy of half or more of all the ancient materials surrounding Mohammad, his life, and the lives and adventures of his companions can readily say that this particular piece of business is simply not proven.
In short, Moslems have to buy the story; non-Moslems can reject it or use it in debates with Moslems, which is alright if you are debating religion, but not alright if you are debating facts.
If we simply move back to the fundamentals - the questions about the legitimacy of those pieces of wood with Arabic writing on them - we can probably resolve 99% of the dilemmas within Islam, and also eliminate the sources of conflict between Islam and Christianity.
Unfortunately for the Moslems, they don't want to ask those questions. After all, in this day and age, to do so could get you killed (and I don't mean like Salman Rushdi because he believes all of this stuff is legitimate and true and faithful to the life and times of the Prophet. He's as bad as the guys who want to kill him.)
In the earliest centuries, Moslems still had it in them to question the legitimacy of text and to differentiate the true from the questionable. This sort of thing hasn't been done for a millenium, and that's why they now find Christians debating just how bad a pedophile their precious Mohammad really was, and using their own "Sunday School Lessons" (Hadiths) to do it.