Because while Truth was a slave herself, she knows exactly what hard situations, slaves went through. Particularly women slaves. This helped her with her speech "Ain't I a Woman" because she went on to explain, that while she was a slave, she could do everything that a man could. Therefore she ended up fighting for women's rights saying that if a woman can do everything a man can, then there should be equal rights...
I hope this helps!!! :D
-Demi
