First of all, endometriosis is a disease of fertile women. Most women with endo who want to have children, have children. Secondly, pregnancy is not a cure for endometriosis. The endometriosis implants generally become less active, and may get smaller and less tender. This seems to be the result of the hormonal changes pregnancy brings.
Menstruation stops, and many women with endometriosis feel much better while they are pregnant.
In addition, an enormous psychological impact occurs. Pregnancy is something a normal, healthy woman's body does. A woman who has suffered with endometriosis and then conceives feels that, finally, she is like other women. She feels normal for perhaps the first time since puberty. It is a liberating, positive experience.