I'm just going to put a guess about what I've read/heard about arranged marriages.
Usually they're used to unite two groups together for a purpose, usually not for love at all. Either for land/resources, power, money, fame, or some other ridiculous greedy reason. Sometimes the people being paired up have no voice in what they truly want, especially when they have alpha family members taking control of their lives. But sometimes they do have a say, but it's usually one-sided.. meaning the person wanting the arranged marriage is making their own decision but is limited to a small selection because of either their own choosing and/or their family's choosing.
I'm pretty sure it still happens today, but not for those reasons as much anymore. Sometimes arranged marriages happen because the families have a somewhat mutual agreement that the couple would look great together and should marry. It's not necessarily subjected to royalty or upper-class like in the olden days..
I don't know. I'm just rambling on what I've read in books, learned in history classes, and watched on TV & movies. xD;