Because it calms the situation many times if you educate them about your personal problems.

For instance when I meet someone for the first time I know I will be around for some time that I have reason to believe does not understand BPD or when I first tell them I have it ... I explain "don't worry, it doesn't mean I am a danger to anyone other than myself at times, and despite its name it is in no way related to DID or bipolar" then I laugh a bit and smile and tell them if they have any questions I don't mind them asking. It generally keeps the rumor mill down and keeps the edge off. Along with allowing me a bit more support at times too ... some of them have asked "are you ok? Is there anything I can do?" when they notice I am having a bad day.