To know when you're really in trouble because the full name gets used (or even more trouble with certain folk who will just start adding names as they shout after you).
I'm told by the time my mother was in her teens, my grandmother had a set pattern of about a dozen names she's use.
There are a lot of different naming systems and traditions, and those where middle names are common developed for different reasons. In some, middle names were where the mother's maiden name or surname of another recent ancestor - so a family name, but not the family name. Also in some communities with fewer surnames, it does help distinguish between people.
There are also communities and cultures where someone has several different names - patronym, other family names, community names, clans - and in attempts to universalize for official documents, these sometimes end up in documentation and treated in that way as middle names. And, as previously mentioned, there is the Christian name and other religious names that in some communities would be added to someone's name.
I have two middle names, neither are the two middle names I was raised with as I changed my entire name as an adult. They're just part of who I am and actually a few years after I changed my name, I realized that the three initials spell another name which I use in my professional life.
My kids all have two middle names, we kinda altered using a religious name, which is common where I'm from - using a Biblical or saint name - to using one with a virtuous meaning we would want them to have, and then for the second, a family name that isn't our surname. So, when I finalized my name change, which was after all my kids were born, I followed the same pattern.