Why is the conversation where Duena Alfonsa tells Grady Cole he can’t marry Alejandra important? PLEASE ANSWER!

I didnt know this conversation was so important until our teacher told us. However, I cant figure out what makes it so significant. I thought Alfonsa was just going on and on about her past and i thought it was almost boring. But, It is not... Can you please tell me why it is so significant. I have a socratic.

Thank You!