Being able to see yourself in a character and relating to them is very important for self esteem
For example, a disabled child who has used crutches for her whole life has never seen a person like her in fictional media. This causes her to think maybe people just hate people like her and maybe something is wrong with her as a person and that's why media seems to hate her. Growing up thinking this leads to low self esteem which can cause all kinds of mental problems including but not limited to: depression, anxiety, eating disorders, inferiority complexes, etc
Also, it's important that people see people that aren't like them alongside characters like them in media. It lets them know that different people exist and they have thoughts and feelings, and it prevents the "you're different from me and are therefore bad" mentality that had caused so many of history's disasters
So yeah. That's the the point of diversity. If you even read it