Multiple personalities are created as a defense mechanism when the person can't deal with a situation that goes on for a long time, the most common example being abused children. Usually the alters (the term for the other personalities) are stronger willed, braver, and protective of "real" person those there is a lot of variation on the last one.
They're considering removing mpd as a recognised disease from the register of mental disorders. Largely because it's suspected that most "personalities" are entirely made up as a way for people to get attention
They've been arguing about that for many many many years. It's not gonna get removed. There are a shit ton of people that legitimately have it. Hell there's probably just as large a ratio of people trying to fake it as there is a ratio of people trying to fake OCD. It's a real disorder with real people affected by it and just like every mental illness a large group of people looking for attention faking it.
HUNDRED
What!? How do you even so many and have not killed someone!?