Why are some girls circumcised? How do they stand the pain?

Illegal mutilation. Female "circumcision" consists of ampuation of clitoris and labia minora. Some of these unfortunate females also have the vaginal opening sewn. More commonly performed on baby or small girls before puberty, and only in a few backward and primitive societies. Poor girls probably scream or pass out from pain and some even die from blood loss. Ugh.
No. This is not an advised procedure and usually done in cultures where women have no choice.
Promiscuity? In misogynistic cultures this practice has been cited as a way to "eliminate promiscuity" by reducing the pleasure of sexual contact. The cutting of clitoris and or labial tissue is the equivalent of a penile and or scrotal amputation in males. Depending on the site, it is often done with no anesthesia by a village member.