Diversity to me from an academic POV is not only racial, but socio-economic.
You can come from Montana: Majorly white, but have have a small percentage of black, Hispanic, Asian, Native Americans.
Have you not met a white kid that lives on food stamps? I am sure you have.
Diversity is as much about ethnicity and race as it has to do with being poor.
To answer your question of why is it good to experience, I will tell you how we taught our children why it was good.
1. When our kids were @ 10,8 and 6 they did the usual whining of why I wouldn't take them to Blockbuster to rent a game. They didn't get they were lucky.
I had enough of their snapping at me. I pulled into a trailer park and said take a look...this is where of some of your classmates live, and they are thankful. I'll give you everything you ask for, but at the same time that means we will move so you can have it. Do you think they are being selfish and placing their needs in front of the family?
Silence. They never ever asked again why they couldn't have a new game or a Xbox/Wii/PS. My kids have 2 yr old cell phones and don't beaach that it isn't an Iphone.
2. We are Catholic, to be confirmed you must do at least 45 hrs+ service/volunteer hours.
Our children did different programs.
DS1 Habitat for Humanity
DD Local soup kitchen
DS2 Church Food pantry
They all saw that race was not a factor, and white, black, asian, hispanic all can fall on hard times.
They learned that they had the same values. They understood that these people had just as much pride as they did, just as intelligent, just as worthy as they were.
They saw past skin color. They saw that experiencing this diversity they now realized they were equals...people who should support each other in time of need.